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/Following.php

@@ -47,6 +47,6 @@ class Following extends BaseModule
 
 		$following = ActivityPub\Transmitter::getContacts($owner, [Contact::SHARING, Contact::FRIEND], 'following', $page);
 
-		System::jsonExit($following, 'application/activity+json');
+		$this->jsonExit($following, 'application/activity+json');
 	}
 }