ActivityPub background actor validation, update and cleanup process (#4765)

* feat(ap): add validation+update+cleanup AP actor job

* fix(test): use time.Time instead of sql.NullTime

* feat(test): add integration test for AP follower cleanup/validation job

activitypub/persistence/followers.go

@@ -19,6 +19,8 @@ func createFederationFollowersTable() {
 		"approved_at" TIMESTAMP,
     "disabled_at" TIMESTAMP,
     "request_object" BLOB,
+    "last_validated_at" TIMESTAMP,
+    "first_validation_failure_at" TIMESTAMP,
 		PRIMARY KEY (iri));`
 	_datastore.MustExec(createTableSQL)
 	_datastore.MustExec(`CREATE INDEX IF NOT EXISTS idx_iri ON ap_followers (iri);`)