[#3863] Refactor: Change Kanban to lazy load the issue data.

Even though this is being cached, I didn't see any performance improvements.
It may be data dependent or there are larger performance issues that should
be addressed later.  But this code reads better so this refactoring is
still good.

app/models/kanban.rb

@@ -20,6 +20,9 @@ def initialize
     @finished_pane = KanbanPane::FinishedPane.new
     @active_pane = KanbanPane::ActivePane.new
     @testing_pane = KanbanPane::TestingPane.new
+
+    @settings = Setting.plugin_redmine_kanban
+    @users = get_users
   end
 
   def self.non_kanban_issues_panes
@@ -39,18 +42,40 @@ def self.staffed_panes
   end
 
   def self.find
-    kanban = Kanban.new
-    kanban.settings = Setting.plugin_redmine_kanban
-    kanban.users = kanban.get_users
-    kanban.incoming_issues = kanban.incoming_pane.get_issues
-    kanban.quick_issues = kanban.quick_pane.get_issues
-    kanban.backlog_issues = kanban.backlog_pane.get_issues(:exclude_ids => kanban.quick_issue_ids)
-    kanban.selected_issues = KanbanIssue.find_selected
-    kanban.active_issues = kanban.active_pane.get_issues(:users => kanban.users)
-    kanban.testing_issues = kanban.testing_pane.get_issues(:users => kanban.users)
-    kanban.finished_issues = kanban.finished_pane.get_issues
-    kanban.canceled_issues = kanban.canceled_pane.get_issues
-    kanban
+    Kanban.new
+  end
+
+  def incoming_issues
+    @incoming_issues ||= incoming_pane.get_issues
+  end
+
+  def quick_issues
+    @quick_issues ||= quick_pane.get_issues
+  end
+
+  def backlog_issues
+    quick_issues # Needs to load quick_issues
+    @backlog_issues ||= backlog_pane.get_issues(:exclude_ids => quick_issue_ids)
+  end
+
+  def selected_issues
+    @selected_issues ||= KanbanIssue.find_selected
+  end
+
+  def active_issues
+    @active_issues ||= active_pane.get_issues(:users => get_users)
+  end
+
+  def testing_issues
+    @testing_issues ||= testing_pane.get_issues(:users => get_users)
+  end
+
+  def finished_issues
+    @finished_issues ||= finished_pane.get_issues
+  end
+
+  def canceled_issues
+    @canceled_issues ||= canceled_pane.get_issues
   end
 
   def get_users