Fix N+1 queries when finding AssignmentRepos and GroupAssignmentRepos

[cyhsutw]

Find these logs when accessing /classrooms/:organization_id/assignments/:id:

 user: cyhsutw
 /classrooms/19820098-google-summer-of-code-2016/assignments/try-swift
 N+1 Query detected
   AssignmentRepo => [:organization]
   Add to your finder: :includes => [:organization]
 N+1 Query method call stack
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/assignment_repo.rb:48:in `github_repository'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/assignment_repo.rb:36:in `disabled?'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/assignment_repos/_assignment_repo.html.erb:1:in `_app_views_assignment_repos__assignment_repo_html_erb__989256760143947223_70262467422220'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/assignments/show.html.erb:34:in `_app_views_assignments_show_html_erb___1588057884647500143_70262486990860'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/assignment_repo.rb:48:in `github_repository'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/assignment_repo.rb:36:in `disabled?'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/assignment_repos/_assignment_repo.html.erb:1:in `_app_views_assignment_repos__assignment_repo_html_erb__989256760143947223_70262467422220'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/assignments/show.html.erb:34:in `_app_views_assignments_show_html_erb___1588057884647500143_70262486990860'

 user: cyhsutw
 /classrooms/19820098-google-summer-of-code-2016/assignments/try-swift
 N+1 Query detected
   AssignmentRepo => [:user]
   Add to your finder: :includes => [:user]
 N+1 Query method call stack
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/assignment_repo.rb:67:in `user'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/assignment_repo.rb:31:in `github_user'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/assignment_repos/_assignment_repo.html.erb:3:in `_app_views_assignment_repos__assignment_repo_html_erb__989256760143947223_70262467422220'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/assignments/show.html.erb:34:in `_app_views_assignments_show_html_erb___1588057884647500143_70262486990860'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/assignment_repo.rb:67:in `user'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/assignment_repo.rb:31:in `github_user'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/assignment_repos/_assignment_repo.html.erb:3:in `_app_views_assignment_repos__assignment_repo_html_erb__989256760143947223_70262467422220'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/assignments/show.html.erb:34:in `_app_views_assignments_show_html_erb___1588057884647500143_70262486990860'

Also, we have these when accessing /classrooms/:organization_id/group-assignments/:id:

 user: cyhsutw
 /classrooms/19820098-google-summer-of-code-2016/group-assignments/fun-with-swift-optionals
 N+1 Query detected
   GroupAssignmentRepo => [:organization]
   Add to your finder: :includes => [:organization]
 N+1 Query method call stack
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/group_assignment_repo.rb:43:in `github_repository'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/group_assignment_repo.rb:39:in `disabled?'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignment_repos/_group_assignment_repo.html.erb:1:in `_app_views_group_assignment_repos__group_assignment_repo_html_erb__1898958708462771443_70262520538040'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignments/show.html.erb:33:in `_app_views_group_assignments_show_html_erb__4258249158097083080_70262537730380'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/group_assignment_repo.rb:43:in `github_repository'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/group_assignment_repo.rb:39:in `disabled?'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignment_repos/_group_assignment_repo.html.erb:1:in `_app_views_group_assignment_repos__group_assignment_repo_html_erb__1898958708462771443_70262520538040'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignments/show.html.erb:33:in `_app_views_group_assignments_show_html_erb__4258249158097083080_70262537730380'

 user: cyhsutw
 /classrooms/19820098-google-summer-of-code-2016/group-assignments/fun-with-swift-optionals
 N+1 Query detected
   GroupAssignmentRepo => [:group]
   Add to your finder: :includes => [:group]
 N+1 Query method call stack
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/group_assignment_repo.rb:47:in `github_team'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/group_assignment_repo.rb:39:in `disabled?'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignment_repos/_group_assignment_repo.html.erb:1:in `_app_views_group_assignment_repos__group_assignment_repo_html_erb__1898958708462771443_70262520538040'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignments/show.html.erb:33:in `_app_views_group_assignments_show_html_erb__4258249158097083080_70262537730380'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/group_assignment_repo.rb:47:in `github_team'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/group_assignment_repo.rb:39:in `disabled?'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignment_repos/_group_assignment_repo.html.erb:1:in `_app_views_group_assignment_repos__group_assignment_repo_html_erb__1898958708462771443_70262520538040'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignments/show.html.erb:33:in `_app_views_group_assignments_show_html_erb__4258249158097083080_70262537730380'

 user: cyhsutw
 /classrooms/19820098-google-summer-of-code-2016/group-assignments/fun-with-swift-optionals
 N+1 Query detected
   Group => [:organization]
   Add to your finder: :includes => [:organization]
 N+1 Query method call stack
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/group.rb:32:in `github_team'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/group_assignment_repo.rb:47:in `github_team'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/group_assignment_repo.rb:39:in `disabled?'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignment_repos/_group_assignment_repo.html.erb:1:in `_app_views_group_assignment_repos__group_assignment_repo_html_erb__1898958708462771443_70262520538040'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignments/show.html.erb:33:in `_app_views_group_assignments_show_html_erb__4258249158097083080_70262537730380'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/group.rb:32:in `github_team'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/group_assignment_repo.rb:47:in `github_team'
   /Users/cyhsutw/Projects/GSoC/classroom/app/models/group_assignment_repo.rb:39:in `disabled?'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignment_repos/_group_assignment_repo.html.erb:1:in `_app_views_group_assignment_repos__group_assignment_repo_html_erb__1898958708462771443_70262520538040'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignments/show.html.erb:33:in `_app_views_group_assignments_show_html_erb__4258249158097083080_70262537730380'

 user: cyhsutw
 /classrooms/19820098-google-summer-of-code-2016/group-assignments/fun-with-swift-optionals
 N+1 Query detected
   RepoAccess => [:user]
   Add to your finder: :includes => [:user]
 N+1 Query method call stack
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignment_repos/_group_assignment_repo.html.erb:9:in `block in _app_views_group_assignment_repos__group_assignment_repo_html_erb__1898958708462771443_70262520538040'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignment_repos/_group_assignment_repo.html.erb:8:in `_app_views_group_assignment_repos__group_assignment_repo_html_erb__1898958708462771443_70262520538040'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignments/show.html.erb:33:in `_app_views_group_assignments_show_html_erb__4258249158097083080_70262537730380'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignment_repos/_group_assignment_repo.html.erb:9:in `block in _app_views_group_assignment_repos__group_assignment_repo_html_erb__1898958708462771443_70262520538040'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignment_repos/_group_assignment_repo.html.erb:8:in `_app_views_group_assignment_repos__group_assignment_repo_html_erb__1898958708462771443_70262520538040'
   /Users/cyhsutw/Projects/GSoC/classroom/app/views/group_assignments/show.html.erb:33:in `_app_views_group_assignments_show_html_erb__4258249158097083080_70262537730380'

This pull request fixes the warnings mentioned above.