Ability to flag inappropriate comments and delete them as admin

Feature description

We can currently flag inappropriate answers for admin review, but we cannot flag comments to answers.

Similarly, as an admin, we can delete other user's answers, but we cannot delete other user's comments.

Use cases

Removing inappropriate or spam comments.

Benefits

Sometimes with many thousands of users you simply cannot prevent a more spammy student to join the instance and start making some inappropriate comment responses to existing answers. Aside from an independent warning to the user, it would be beneficial to also be able to track down those comments and remove them, based on raised flags.

Requirements

New database field (and migration) for CommentMixin to have a flagged count as well, and a corresponding UI for it.

Links / References / Examples

N/A. For the privacy of the user I'll refrain from posting a direct screenshot of the inappropriate comment (this is on the University of Edinburgh instance). But it's very easily reproducible on a test instance.