Use span for merge request reference

Cite isn't semantically correct in this case.

Styling for `cite` is also removed here.

app/assets/stylesheets/pages/issuable.scss

@@ -238,10 +238,6 @@
       text-overflow: ellipsis;
     }
 
-    cite {
-      font-style: normal;
-    }
-
     button {
       float: right;
       padding: 1px 5px;

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

@@ -165,6 +165,6 @@
         .cross-project-reference.hide-collapsed
           %span
             = s_('MilestoneSidebar|Reference:')
-            %cite{ title: milestone_ref }
+            %span{ title: milestone_ref }
               = milestone_ref
           = clipboard_button(text: milestone_ref, title: s_('MilestoneSidebar|Copy reference'), placement: "left", boundary: 'viewport')