Add shared inbox support for ActivityPub delivery (#4755)

* feat(ap): add support for shared inboxes to reduce outbound load

* feat(db): refactor ap followers db into followers repository

* fix(ap): use the updated activity library to pull out the shared inbox endpoint

* chore(deps): point at updated build of owncast/activity

* fix(ap): typeless endpoints

* feat(test): update ActivityPub test to support shared inboxes

* chore(test): remove unused variable

* fix: feedback from review. Guard against SSRF/non-HTTPS/local and handle transaction errors

activitypub/resolvers/follow.go

@@ -29,6 +29,7 @@ func MakeFollowRequest(c context.Context, activity vocab.ActivityStreamsFollow)
 		ActorIri:         person.ActorIri,
 		FollowRequestIri: activity.GetJSONLDId().Get(),
 		Inbox:            person.Inbox,
+		SharedInbox:      person.SharedInbox,
 		Name:             person.Name,
 		Username:         fullUsername,
 		Image:            person.Image,