Hide registration form if max registration reached

See https://github.com/FreshRSS/FreshRSS/issues/679

app/Controllers/authController.php

@@ -351,5 +351,8 @@ class FreshRSS_auth_Controller extends Minz_ActionController {
 	 * This action gives possibility to a user to create an account.
 	 */
 	public function registerAction() {
+		if (max_registrations_reached()) {
+			Minz_Error::error(403);
+		}
 	}
 }