Remove ugly userrights-CentralAuth hack introduced in r48970 and friends:

Remove ugly userrights-CentralAuth hack introduced in r48970 and friends:
* Make UserrightsForm::doSaveUserGroups(), addLogEntry() and helpers non-static again so CentralAuth can override them; remove the short-lived UserRightsLogEntry hook
* Let UserrightsForm::fetchUser() return a WikiErrorMsg on failure
* In ApiUserrights, use an instance of the UserrightsPage class rather than calling its methods statically. This also enables interwiki userrights in this module
* Add some messages to ApiBase::$messageMap