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
I looked into its impl, it just clears a few arrays so shoulnd't be expensive.
Also we don't do it every time automatically, the test writers must decide whether to use it or not. (encryption/sharing have their own helpers which can be redirected to this later)