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
Just asked @icewind1991 again he said it depends on the use case.
For `searchByTags()` it is less cumbersome for the API consumer to just pass a string.

If the inconsistency still makes you unhappy I can change TagManager to use strings instead.