Allow wrapping of note headline

This prevents a horizontal scrollbar from appearing in Firefox in
certain situations. See the [merge request][1] for more details and
discussion.

[1]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25001#note_141208011

app/assets/stylesheets/pages/notes.scss

@@ -602,7 +602,6 @@ $note-form-margin-left: 72px;
 .note-headline-meta {
   display: inline-block;
-  white-space: nowrap;
   .system-note-message {
     white-space: normal;