Drop default ORDER scope when calling a find method on a Sortable model

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/23079 and https://gitlab.com/gitlab-org/gitlab-ce/issues/34412

Replaces https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8662

/cc @stanhu @yorickpeterse @to1ne @zj

app/controllers/projects/issues_controller.rb

@@ -227,7 +227,7 @@ class Projects::IssuesController < Projects::ApplicationController
   def issue
     return @issue if defined?(@issue)
     # The Sortable default scope causes performance issues when used with find_by
-    @noteable = @issue ||= @project.issues.where(iid: params[:id]).reorder(nil).take!
+    @noteable = @issue ||= @project.issues.find_by!(iid: params[:id])
 
     return render_404 unless can?(current_user, :read_issue, @issue)