* API: (bug 15935) Add action=userrights to the API

* API: (bug 15935) Add action=userrights to the API
* Add ustoken=userrights to list=users
* Move the non-UI part of UserrightsPage::saveUserGroups() to the static and more generic doSaveUserGroups()
* Add a $reason parameter to UserrightsPage::addLogEntry() and make it and its helpers static
* Move UserrightsPage::changeableGroups() and changeableByGroup() to the User class and make the latter static
* In doSaveUserGroups(), drop groups that the user doesn't have from $remove (and those that they do have from $add), and return array($add, $remove)
* Fix up a comment in ApiQueryRecentChanges