Add support for mentions

[WowSuchRicky]

When I tag a user via the bridged matrix room (e.g. @User) they are tagged and it is clickable on the Matrix side. This used to also work on the FB side. However, it no longer does, and it just sends the text "User (FB)" to them as a message instead.

Let me know if I can add any actual useful data points for you to debug or logs etc.

[tulir]

That's already listed in the roadmap file