Render links in edit comments

[cellio]

meta:293096

Edits in a post's history can include comments, and occasionally those comments include links (for example to answers or GitHub issues). It would be helpful, especially on phones, if we rendered those links instead of making you select and copy/paste.

If we do this, we need to somehow handle the problem of malicious links. (It's entirely possible that a spammer would propose a good edit with a bad comment, and the edit reviewer would miss the comment while looking at the diff.) Rather than trying to design some sort of whitelist scheme for links, I suggested #1543 to allow comments to be edited ex-post-facto. After we do that we could allow rendering.