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

- [ ] REQUIRES: getTagsForObjects https://github.com/owncloud/core/pull/12409
- [ ] TODO: add unit test for sharedstorage
check whether we are already logged out, since this might be expensive?