Autolink URLs in message popups on /flagged page

[przemoc]

It would be nice to have autolinking URLs in message popups, so we could simply click such link. Bells and whistles kind of thing, I guess, but would be useful for people browsing /flagged that are not maintainers (as maintainers get messages in their mailboxes).

Remember to use rel="nofollow" in a tag if ever implementing that.

[clandmeter]

Not sure i want to implement this from the backend. Maybe something like https://github.com/markdown-it/linkify-it would be better.