* Fix ActivityPub handling of replies when LOCAL_DOMAIN ≠ WEB_DOMAIN (#4895)
For all intents and purposes, `local_url?` is used to check if an URL refers
to the Web UI or the various API endpoints of the local instances. Those things
reside on `WEB_DOMAIN` and not `LOCAL_DOMAIN`.
* Change local_url? spec, as all URLs handled by Mastodon are based on WEB_DOMAIN