[feature/oauth] Create means to associate phpBB account with external

PHPBB3-11673

phpBB/phpbb/auth/provider/oauth/service/bitly.php

@@ -76,6 +76,7 @@ class phpbb_auth_provider_oauth_service_bitly extends phpbb_auth_provider_oauth_
 		// Send a request with it
 		$result = json_decode( $this->service_provider->request('user/info'), true );
 
-		// Get the user id
+		// Return the unique identifier returned from bitly
+		return $result['data']['login'];
 	}
 }