Add default `markdown_version`

ee/app/views/groups/epics/_discussion.html.haml

@@ -3,4 +3,5 @@
 %section.js-vue-notes-event
   #js-vue-notes{ data: { notes_data: notes_data(@epic),
     noteable_data: EpicSerializer.new(current_user: current_user).represent(@epic).to_json,
-    current_user_data: UserSerializer.new.represent(current_user, only_path: true).to_json, noteable_type: 'epic' } }
+    current_user_data: UserSerializer.new.represent(current_user, only_path: true).to_json, noteable_type: 'epic',
+    markdown_version: @issuable.cached_markdown_version } }