Show assignees in the merge-requests index

app/views/projects/merge_requests/_merge_request.html.haml

@@ -15,8 +15,8 @@
         = merge_request.target_branch
   .merge-request-info
     %span.light= "##{merge_request.iid}"
-    - if merge_request.author
-      authored by #{link_to_member(merge_request.source_project, merge_request.author)}
+    - if merge_request.assignee
+      assigned to #{link_to_member(merge_request.source_project, merge_request.assignee)}
     - if merge_request.votes_count > 0
       = render 'votes/votes_inline', votable: merge_request
     - if merge_request.notes.any?