Move System::jsonExit to BaseModule->jsonExit

- This will ensure headers set in BaseModule->run will be carried in jsonExit scenarios
- Deprecate jsonExit() method in Core\System

src/Module/ActivityPub/Inbox.php

@@ -66,7 +66,7 @@ class Inbox extends BaseApi
 			$inbox = ActivityPub\ClientToServer::getPublicInbox($uid, $page, $request['max_id'] ?? null);
 		}
 
-		System::jsonExit($inbox, 'application/activity+json');
+		$this->jsonExit($inbox, 'application/activity+json');
 	}
 
 	protected function post(array $request = [])