Probably nice for the people that contributed to 9.0 to see themselves in the AUTHORS file :)

This commit consists of patches automatically generated for this project on https://scrutinizer-ci.com

* Use query builder
* Minor unit tests additions

This avoids duplicated queries like first checking the group_users db
and then just doing a select on the group db. Those enries are linked
(and should be using foreign keys!)

This commit makes sure we cache those entries.
If a user is part of N groups this saves N queries on webdav access

* DI
* Tests
* moved OC_SubAdmin to legacy
* Added to private OC\GroupManager

This is a string

This reverts commit 6a1a4880.

First step of a somewhat testable user management. - I know, the JSON returns are in an ugly format but the JS expects it that way. So let's keep it that way until we have time to fix the JS in the future.