adds avatar for discussion note

app/views/discussions/_discussion.html.haml

 - expanded = discussion.expanded?
 %li.note.note-discussion.timeline-entry
   .timeline-entry-inner
+    .timeline-icon
+      = link_to user_path(discussion.author) do
+        = image_tag avatar_icon(discussion.author), class: "avatar s40"
     .timeline-content
       .discussion.js-toggle-container{ class: discussion.id, data: { discussion_id: discussion.id } }
         .discussion-header