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
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
Next step in server-to-server sharing next generation, see #12285

Beside some small improvements and bug fixes this will probably the final state for OC8.

To test this you need to set up two ownCloud instances. Let's say:

URL: myPC/firstOwnCloud user: user1
URL: myPC/secondOwnCloud user: user2
Now user1 can share a file with user2 by entering the username and the URL to the second ownCloud to the share-drop-down, in this case "user2@myPC/secondOwnCloud".

The next time user2 login he will get a notification that he received a server-to-server share with the option to accept/decline it. If he accept it the share will be mounted. In both cases a event will be send back to user1 and add a notification to the activity stream that the share was accepted/declined.

If user1 decides to unshare the file again from user2 the share will automatically be removed from the second ownCloud server and user2 will see a notification in his activity stream that user1@myPC/firstOwnCloud has unshared the file/folder from him.