Ability to delete a post diff from its history

[julianlam]

Available to moderators (of that pid's category), gmods, and administrators only (hint: privileges.users.isModerator), allow deletion of a post's history entry (one at a time).

Suggested API route to add: DELETE /api/v3/posts/:pid/diffs/:timestamp

Caveat: The way post diffs work, every time a post is edited, a diff is made from the old version to the new version (or perhaps it is the other way around). This minimizes the amount of data we store, but means we cannot arbitrarily delete diffs in the middle, because it breaks the chain and older revisions will no longer be able to be restored. The easiest solution may just be to soft delete them internally, and not show them in the frontend (nor can it be restored if requested).

Alternatively if you really want to make them permanent, you could re-generate the older diffs upon deletion. Up to you.