Speed up dashboard milestone index by scoping IssuesFinder to user authorized projects

It improves performance in dashboard milestone index page by passing a
hint to "IssuesFinder". "IssuesFinder" generates a more performant query
when it is limited to authorized projects for user.
In the dashboard we already limit the projects to these authorized for
user (see "Dashboard::ApplicationController#projects"), so we can safely
pass this option to "IssuesFinder".

app/controllers/dashboard/milestones_controller.rb

@@ -19,11 +19,11 @@ class Dashboard::MilestonesController < Dashboard::ApplicationController
   private
 
   def milestones
-    @milestones = GlobalMilestone.build_collection(@projects, params)
+    @milestones = DashboardMilestone.build_collection(@projects, params)
   end
 
   def milestone
-    @milestone = GlobalMilestone.build(@projects, params[:title])
+    @milestone = DashboardMilestone.build(@projects, params[:title])
     render_404 unless @milestone
   end
 end

app/models/dashboard_milestone.rb

+class DashboardMilestone < GlobalMilestone
+  def issues_finder_params
+    { authorized_only: true }
+  end
+end

changelogs/unreleased/speed-up-dashboard-milestone-index.yml

+---
+title: Speed up dashboard milestone index by scoping IssuesFinder to user authorized
+  projects
+merge_request: 8524
+author: