Add admin_merge_request ability to Developer level and up

This was necessary because this permission is checked dynamically by
`app/views/shared/issuable/_context` when on a Merge Request.

Closes #2058

app/models/ability.rb

@@ -158,12 +158,13 @@ class Ability
         :create_project_snippet,
         :update_issue,
         :admin_issue,
-        :admin_label,
+        :admin_label
       ]
     end
 
     def project_dev_rules
       project_report_rules + [
+        :admin_merge_request,
         :create_merge_request,
         :create_wiki,
         :push_code