Allow or disallow comment replies to an event ActivityPub representation

Currently uses our dirty proprietary property:

"repliesModerationOption": "allow_all"|"moderated"|"closed"

See the ActivityPub issue for why there's no way to do this.

Things we can probably explore in the future:

• Litepub Capability Enforcement (seems abandoned)
• OcapPub
• Bearcaps (thread on SocialHub)