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
This change will expose the user backend via the REST API which is a pre-requisite for https://github.com/owncloud/core/issues/12620.
For example:
````json
[{"name":"9707A09E-CA9A-4ABE-A66A-3F632F16C409","displayname":"Document Conversion User Account","groups":[],"subadmin":[],"quota":"default","storageLocation":"\/Users\/lreschke\/Programming\/core\/data\/9707A09E-CA9A-4ABE-A66A-3F632F16C409","lastLogin":0,"backend":"OCA\\user_ldap\\USER_LDAP"},{"name":"ED86733E-745C-4E4D-90CB-278A9737DB3C","displayname":"Hacker","groups":[],"subadmin":[],"quota":"default","storageLocation":"\/Users\/lreschke\/Programming\/core\/data\/ED86733E-745C-4E4D-90CB-278A9737DB3C","lastLogin":0,"backend":"OCA\\user_ldap\\USER_LDAP"},{"name":"71CDF45B-E125-450D-983C-D9192F36EC88","displayname":"admin","groups":[],"subadmin":[],"quota":"default","storageLocation":"\/Users\/lreschke\/Programming\/core\/data\/71CDF45B-E125-450D-983C-D9192F36EC88","lastLogin":0,"backend":"OCA\\user_ldap\\USER_LDAP"},{"name":"admin","displayname":"admin","groups":["admin"],"subadmin":[],"quota":"default","storageLocation":"\/Users\/lreschke\/Programming\/core\/data\/admin","lastLogin":"1418057287","backend":"OC_User_Database"},{"name":"test","displayname":"test","groups":[],"subadmin":[],"quota":"default","storageLocation":"\/Users\/lreschke\/Programming\/core\/data\/test","lastLogin":0,"backend":"OC_User_Database"}]
```
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.
