Add REST route for managing groups

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:
- [x] Migrate get handler (index)
- [x] Migrate create handler (create)
- [x] Migrate destroy handler (destroy)
- [ ] Add show route (show)
- [ ] Add possibility to rename group (update)

FIXME:
- [ ] Fix JS code for deletion of groups

Requires https://github.com/owncloud/core/pull/12619

@MorrisJobke 
80 chars please - THX