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/pull/10697/files. As you can see we added a explicit setupFS() to the delete user operation. We probably need something similar for the create user operation to make sure that the filesystem is initialized if a 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?