Reimplement commenting on image diffs

This stems from the Merge Request refactor effort (#48034 (moved)). Upon launch, some features had been disabled/lost and need to be recovered.

10.8.x 11.1 RC4 (Currently deployed on gitlab.com)
2018-07-06_09.54.57 2018-07-06_09.55.10

Previously, clicking on an image diff would allow you to create a discussion.

The image frames are not displaying image comments despite some having them.

Tasks:

  • display image comments when present
  • reenable way to add comments to images
Edited by James Ramsay (ex-GitLab)