Enhancement: Introduce user-management app

We've added the app "user-management" with the following features:
* Listing, creating, deleting and editing users
* Listing, creating, deleting and editing group
* Universal search in users and groups view

https://github.com/owncloud/web/issues/6673
https://github.com/owncloud/web/issues/6674
https://github.com/owncloud/web/issues/6675
https://github.com/owncloud/web/issues/6676
https://github.com/owncloud/web/issues/6677
