Move System::httpExit to BaseModule->httpExit

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

src/Module/OStatus/PubSub.php

@@ -155,6 +155,6 @@ class PubSub extends \Friendica\BaseModule
 			$this->logger->notice('Success for contact.', ['mode' => $hub_mode, 'contact' => $contact_id]);
 		}
 
-		System::httpExit($hub_challenge);
+		$this->httpExit($hub_challenge);
 	}
 }