Support rollbacks in edit history

[cellio]

https://meta.codidact.com/questions/279795

If you need to undo an edit, you have to do it by hand. Let's add rollback support to make that easier.

Suggested presentation: in the edit history for a post, if and only if you have the edit ability, show something with each non-current revision that says "roll back to this version". If the user clicks that, prompt for an edit reason (like for other edits) to be appended to the standard notice: "rolled back to version N (version timestamp)".

Notes:

1. I'm suggesting we gate it by the edit ability to avoid having rollbacks as suggested edits, which seems like it could be confusing to review. If you can't edit you can't roll back, sorry. You can always leave a comment or raise a flag if there's something that needs attention.

2. The edit history should clearly show that an edit was a rollback no matter what the person does or doesn't type, hence the standard text there (which can be supplemented). I'm suggesting that we include the version number of the edit being rolled back to, for easier history viewing, and the timestamp of that revision, so it'll be more obvious if somebody just rolled a post back by a year or something.

3. Yeah, that would require revision numbers in the edit history, which we don't have. Can we?