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"}]
```
* keep old static methods - mapped to new ones and deprecated
* removed deleteApp, getUsers, getApps because they are unused
* make AllConfig unit tests more robust against not cleaned up environments
* introduce SystemConfig to avoid DI circle (used by database connection which is itself needed by AllConfig that itself contains the methods to access the config.php which then would need the database connection - did you get it? ;))
* use DI container and use that method in legacy code paths (for easier refactoring later)
* create and use getSystemConfig instead of query() in DI container
Otherwise the headers from `JSONResponse` are gone and the Content-Type of the response would be `text/html` instead of `application/json; charset=utf-8`. This leads to broken scripts since we set the `nosniff` tag, furthermore this is very bad from a security PoV.
