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

persistence/userrepository/userrepository.go

@@ -144,7 +144,7 @@ func (r *SqlUserRepository) ChangeUserColor(userID string, color int) error {
 	defer r.datastore.DbLock.Unlock()
 
 	if err := r.datastore.GetQueries().ChangeDisplayColor(context.Background(), db.ChangeDisplayColorParams{
-		DisplayColor: color,
+		DisplayColor: utils.SafeIntToInt32(color),
 		ID:           userID,
 	}); err != nil {
 		return errors.Wrap(err, "unable to change display color")