make sure that the users file system is initialized before we emit the post hook

Because I don't can reproduce issue https://github.com/owncloud/core/issues/11289 that's just an idea.

This PR probably introduced the problem https://github.com/owncloud/core/issues/11289. As you can see we added a explicit setupFS() to the delete user hook. We probably need something similar for the create user hook to make sure that the filesystem is initialized if any app wants to react on the event and for example place some data to the users data directory.

cc @DeepDiver1975  @icewind1991  what do you think? Does it makes sense?