Merge branch '5941-extract-ee-controllers' into 'master'

Backport of "Extract EE specific files/lines for app/controllers/groups"

See merge request gitlab-org/gitlab-ce!19343

app/controllers/groups/group_members_controller.rb

@@ -3,8 +3,12 @@ class Groups::GroupMembersController < Groups::ApplicationController
   include MembersPresentation
   include SortingHelper
 
+  def self.admin_not_required_endpoints
+    %i[index leave request_access]
+  end
+
   # Authorize
-  before_action :authorize_admin_group_member!, except: [:index, :leave, :request_access]
+  before_action :authorize_admin_group_member!, except: admin_not_required_endpoints
 
   skip_cross_project_access_check :index, :create, :update, :destroy, :request_access,
                                   :approve_access_request, :leave, :resend_invite,

app/controllers/groups/milestones_controller.rb

@@ -76,12 +76,15 @@ class Groups::MilestonesController < Groups::ApplicationController
 
   def milestones
     milestones = MilestonesFinder.new(search_params).execute
-    legacy_milestones = GroupMilestone.build_collection(group, group_projects, params)
 
     @sort = params[:sort] || 'due_date_asc'
     MilestoneArray.sort(milestones + legacy_milestones, @sort)
   end
 
+  def legacy_milestones
+    GroupMilestone.build_collection(group, group_projects, params)
+  end
+
   def milestone
     @milestone =
       if params[:title]