Suggest Facebook when adding account

[Manouchehri]

Despite Facebook claiming they were going to shut XMPP down (#658), it's still quite alive and well. Only xmpp_login from the API was removed, not the actual service. (I've heard reports that you need a username and cannot login with an ID, unconfirmed though.)

A user can just add chat.facebook.com along with their username already, but I think it would be a good idea to simplify the process a little. Adding a template similar to the Google one would likely improve usability for non-technical users.

Thoughts?