Allow moderators to edit revision comments

[cellio]

Sometimes a good edit (or suggested edit) is accompanied by an edit comment that is not so nice. Let's allow moderators to edit these comments from the post history. The history should indicate somehow that the comment was edited (otherwise we're putting words in the editor's mouth), and ideally moderators would be able to see the original.

This request is low priority, but I think a prerequisite for rendering links in comments as requested in meta:293096.