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/CustomEmojis.php

@@ -41,6 +41,6 @@ class CustomEmojis extends BaseApi
 	{
 		$emojis = DI::mstdnEmoji()->createCollectionFromSmilies(Smilies::getList());
 
-		System::jsonExit($emojis->getArrayCopy());
+		$this->jsonExit($emojis->getArrayCopy());
 	}
 }