Add REST route for managing groups and users

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.

Todo (group):
- [x] Migrate get handler (index)
- [x] Migrate create handler (create)
- [x] Migrate destroy handler (destroy)
- [ ] ~~Add possibility to rename group (update)~~ (requires more work - let's do this later)
- [x] Write unit tests  
- [x] Fix for subadmins

Todo (user):
- [x] Migrate get handler (index)
- [x] Migrate create handler (create)
- [x] Migrate destroy handler (destroy)
- [x] Write unit tests
- [x] Fix for subadmins

FIXME:
- [x] Fix JS code for deletion of groups

Requires https://github.com/owncloud/core/pull/12619

@MorrisJobke 
@icewind1991 Your feedback on that, please. THX!