Add REST route for user & group management

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.
fix updated URL schema in JS unit tests
Fix url
Add statuscodes
Merge branch 'initial-work-migrate-to-appframework' of https://github.com/owncloud/core into initial-work-migrate-to-appframework
React on other statuscodes than 200
Merge pull request #12618 from owncloud/initial-work-migrate-to-appframework

Add REST route for managing groups and users