Add filter for 'backend' to user REST route

This adds a "backend" type filter to the index REST route which is a pre-requisite for https://github.com/owncloud/core/issues/12620

For example when calling `index.php/settings/users/users?offset=0&limit=10&gid=&pattern=&backend=OC_User_Database` only users within the backend `OC_User_Database` would be shown. (requires sending a CSRF token as well)

Depends upon https://github.com/owncloud/core/pull/12711
Break loop
Use public interface
Add test for user without backend
Use array key instead of value
Use limit and offset
Comment code path
Make comment clear
Merge pull request #12726 from owncloud/add-filter-for-backend-to-rest-index

Add filter for backend to rest index