Add support for linking XMPP URIs in toots (#12709)

Add support for linking XMPP URIs in toots (#12709)

* Fix wrong grouping in Twitter valid_url regex

* Add support for xmpp URIs

Fixes #9776

The difficult part is autolinking, because Twitter-text's extractor does
some pretty ad-hoc stuff to find things that “look like” URLs, and XMPP
URIs do not really match the assumptions of that lib, so it doesn't sound
wise to try to shoehorn it into the existing regex.

This is why I used a specific regex (very close, although slightly more
permissive than the RFC), and a specific scan function (a simplified version
of the generalized one from Twitter).

* Remove leading “xmpp:” from auto-linked text