Allow webfinger controller to reply to user@WEB_DOMAIN in addition to user@LOCAL_DOMAIN (#2351)

This provides a hotfix for outbound salmon requests to other Mastodon instances
as they currently will try to resovle user@WEB_DOMAIN instead of user@LOCAL_DOMAIN
(see #2012 and #20312).

Furthermore, this should ease transition from users switching from
LOCAL_DOMAIN = WEB_DOMAIN to another LOCAL_DOMAIN when WEB_DOMAIN does not change.