adds exclamation point when assigned user cannot merge

app/assets/stylesheets/pages/projects.scss

@@ -558,3 +558,8 @@ pre.light-well {
     width: 101%;
   }
 }
+
+.cannot-be-merged, .cannot-be-merged:hover {
+  color: #E62958;
+  margin-top: 2px;
+}

app/views/shared/issuable/_sidebar.html.haml

@@ -10,6 +10,9 @@
       .value
         - if issuable.assignee
           %strong= link_to_member(@project, issuable.assignee, size: 24)
+          - if issuable.instance_of?(MergeRequest) && !@merge_request.can_be_merged_by?(issuable.assignee)
+            %a.pull-right.cannot-be-merged{href:'#', data:{toggle:'tooltip'}, title:'Not allowed to merge'}
+              = icon('exclamation-triangle')
         - else
           .light None