* Allow running mastodon on a different domain as the one used for identifying users
* Alter documentation of WEB_DOMAIN to make clear it shouldn't be used unless the admin knows what they are doing
* Compare to web_domain instead of local_domain when dealing with feeds/API
* Correctly identify mentions to local accounts
Mentions URLs point to the person's web profile, i.e., the user page served on WEB_DOMAIN.
