Show Milestone information in MR Listing

When a MR belongs to a Milestone, show this information in the listing
with a small box telling the Milestone title.

app/views/merge_requests/_merge_request.html.haml

@@ -10,6 +10,10 @@
         %span.btn.small.disabled.grouped
           %i.icon-comment
           = merge_request.mr_and_commit_notes.count
+      - if merge_request.milestone_id?
+        %span.btn.small.disabled.grouped
+          %i.icon-time
+          = merge_request.project.milestones.find(merge_request.milestone_id).title
       %span.btn.small.disabled.grouped
         = merge_request.source_branch
         →