Load related MRs/branches asynchronously

Currently this works by loading the HAML partials via XHR. While this is
not the nicest setup it _is_ the easiest setup using the tools we
currently have.

Loading this data asynchronously doesn't make loading the related
MRs/branches itself faster, it merely ensures that loading the issue
itself is not slowed down.

Fixes gitlab-org/gitlab-ce#14949

config/routes.rb

@@ -701,6 +701,8 @@ Rails.application.routes.draw do
         resources :issues, constraints: { id: /\d+/ } do
           member do
             post :toggle_subscription
+            get :referenced_merge_requests
+            get :related_branches
           end
           collection do
             post  :bulk_update