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/Api/Mastodon/Statuses/RebloggedBy.php

@@ -56,6 +56,6 @@ class RebloggedBy extends BaseApi
 			$accounts[] = DI::mstdnAccount()->createFromContactId($activity['author-id'], $uid);
 		}
 
-		System::jsonExit($accounts);
+		$this->jsonExit($accounts);
 	}
 }