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.
The following screenshot illustrates the issue:
See merge request !956
