This leverages the new BlockedExternalDomains system that is now part of
AbuseFilter. It notifies editors in realtime if a link they add is
blocked. See https://w.wiki/7ZsF for more information.
BlockedExternalDomains is slated to have its own API tantamount to the
action=spamblacklist endpoint, after which case this code will need to be
updated. In the meantime, it's meant to serve as a minimal viable product
for the CWS 2023 wish <https://w.wiki/7ZsE> for wikitext users.
The new $wgAbuseFilterBlockedExternalDomainsNotification configuration
setting controls the availability of this feature.
A similar feature for VisaulEditor is tracked at T276857
Bug: T347435
Change-Id: I7eae55f12da9ee58be5786bfc153e549b09598e7
