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
Move file thumbnail into the label element
Replace input checkbox CSS selector with .electCheckBox
Add test for user without backend
Show user backend in user management

* add switch to settings to show user backend
* user classes for headers and rows to unify the show/hide statement
* add user backend to response of user create request
* proper markup in settings area
Merge pull request #12806 from owncloud/display-backend-in-usermgnt

Show user backend in user management
Use array key instead of value
Use limit and offset
Comment code path
Make comment clear
Return ? if mtime is 0 or -1

Tooltip contains 'Unable to determine date'. Fixes #6395
[tx-robot] updated from transifex
Merge pull request #12726 from owncloud/add-filter-for-backend-to-rest-index

Add filter for backend to rest index
Merge pull request #12795 from owncloud/files-layoutchanges

Move file thumbnail element into the label tag
Merge pull request #12639 from owncloud/bad_mtime_format

Fix formatting of bad dates from external storages
[tx-robot] updated from transifex
make regex in controllermethodreflector.php compatible with PCRE 6.x

The syntax ?<...> seems to be only supported from PCRE 7.0 on. For
backwards-compability ?P<...> is used.
[tx-robot] updated from transifex