Support Mastodon instances where WEB_DOMAIN != LOCAL_DOMAIN

[stiell]

I have an instance where WEB_DOMAIN (where the Mastodon API endpoint resides) is different from LOCAL_DOMAIN (the part used for the Mastodon identifier). If I try signing in from the bridge using LOCAL_DOMAIN, the bridge app returns a HTTP 500 response. Signing in using WEB_DOMAIN works, but this gives me the wrong Mastodon identifier.