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/Objects.php

@@ -130,6 +130,6 @@ class Objects extends BaseModule
 		// Relaxed CORS header for public items
 		header('Access-Control-Allow-Origin: *');
 
-		System::jsonExit($data, 'application/activity+json');
+		$this->jsonExit($data, 'application/activity+json');
 	}
 }