New permissions framework

[rafalp]

New permission system for Misago. Will exist in parallel to old one until all features are using it exclusively.

Fixes #1527

TODO

• Datamodel
  • User model
  • Group model
• Admin auth
  • Use misago.User.is_misago_admin for misago admin status
  • Use misago.User.is_misago_root for misago super-admin status
  • Update admin auth logic checking to ignore is_staff and is_superuser
  • Update admin auth tests
• Groups admin
  • List groups
  • Reorder groups
  • Create group
  • Copy new group's permissions from other group
  • Edit group
  • Edit group category permissions
  • Copy existing group's permissions from other group
  • Make a default
  • Delete group
  • View members
  • Invalidate groups cache
• Users admin
  • Create user with groups
  • Edit user groups
  • Display user main group on list
  • Search users with main group
  • Search users with any group
  • Show admin users by group
• Categories admin
  • Copy category permissions
  • Change category permissions
• Moderators admin
  • List moderators
  • Add moderator
  • Edit moderator
  • Delete moderator
• Add "Deprecated" warnings to old admin pages
  • Ranks
  • Permissions
  • Category permissions
  • Users
• Permissions factory
  • Get permissions
  • Cache permissions
  • Build permissions
  • Build category permissions
  • Lazy user permissions proxy
  • Debug Toolbar Tab
• Auth (other)
  • Exclude user.is_misago_admin from ban checks
  • Replace user.is_staff in special conditions with user.is_misago_admin
• Tests
  • Check staffuser fixture use in tests
  • Check superuser fixture use in tests
• Update fake_category factory to set default permissions

[coveralls]

coverage: 98.649% (+0.06%) from 98.587%
when pulling 91ce26b on fix-1527-new-permission-system
into 0c99ee3 on main.