Check that intro is startable for given app on backend side

Currently checkIntroStartable is called after getIntrosStructure and getCurrentLanguageIntro are called.

It would be nice to make this check on backend side instead: using the same logic as the one currently in main.js, something like:

$url = $this->request->getRequestUri();
// pattern match to get the app
// check the config user value to see if "introjs-dontShowAgain-$app" exists
// set an initial state that can be handled in front-end https://docs.nextcloud.com/server/latest/developer_manual/basics/front-end/js.html#loading-initial-state to show the intro or not

It's possible that I'm missing something that prevents you to do this that way.