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/Accounts/Following.php

@@ -115,6 +115,6 @@ class Following extends BaseApi
 		}
 
 		self::setLinkHeader();
-		System::jsonExit($accounts);
+		$this->jsonExit($accounts);
 	}
 }