Improve the scenario of adding a comment on deleted images in MR diffs

Problem to solve

When we reimplemented the commenting on images of MR diffs, we didn't cover this scenario.

We can see this here: https://gitlab.com/gitlab-org/design.gitlab.com/merge_requests/214/diffs

We either implement the commenting or don't make the UI suggest adding the comment, whichever is preferable.

Intended users

Any user commenting on MR diffs with images, essencial for docs, UX, etc.

Proposal

Cover the scenario of deleted images, whether by:

1. implement commenting on deleted images

or

2. don't display a comment bubble on deleted / give a visual indication that you can't commented on deleted.

Whatever makes more sense for UX/Product.