Fix file upload dialog for comment editing

This fixes the file upload dialog which currently works for new comments
but not for comment editing. The cause for this is a missing DOM class
which causes traversal to the dropzone to fail.

This is fixed by adding the expected `gfm-form` class to the edit form.
It has to be added dynamically since adding it to the form template
would break the form's layout and comment saving.

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.14.0 (unreleased)
+  - Fix file upload dialog for comment editing (Daniel Gerhardt)
   - Expire Rails cache entries after two weeks to prevent endless Redis growth
   - Add support for destroying project milestones (Stan Hu)
 

app/assets/javascripts/notes.js.coffee

@@ -298,7 +298,7 @@ class @Notes
     note.find(".note-header").hide()
     base_form = note.find(".note-edit-form")
     form = base_form.clone().insertAfter(base_form)
-    form.addClass('current-note-edit-form')
+    form.addClass('current-note-edit-form gfm-form')
     form.find('.div-dropzone').remove()
 
     # Show the attachment delete link