Added searchByTags to view, storage and cache

Needed to be able to get a flat result list of all favorites.

@icewind1991 @schiesbn @nickvergessen please review

- [x] REQUIRES: getTagsForObjects https://github.com/owncloud/core/pull/12409
- [x] TODO: add unit test for sharedstorage
Note that manually creating the user requires you to do `new \OC\User\User($userId, null)` at the moment, `null` being ugly and having to use the private namespace is ugly too.