Fix EZP-23403: Fix implementation of Persistence\Solr\Slot\MoveUserGroup slot

This PR resolves issue https://jira.ez.no/browse/EZP-23403

It turned out `MoveSubtree` slot also need to be fixed, as it was reindexing all Locations of the Content in the moved subtree, while only moved Locations needed reindexing.
`MoveUserGroup` slot is fixed by extending `MoveSubtree` slot and using its implementation.

Testing: manual testing

Followup: remove UserService::moveUserGroup() method: https://jira.ez.no/browse/EZP-23699
I think `indexSubtree()` would fit better. Will update.