Add User::isEveryoneAllowed function

User::groupHasPermission is used for various purposes, from checking
whether it makes sense to show a "hide logged-in users" on
Special:NewPages to showing different error messages in some places when
'user' or 'autoconfirmed' is allowed the action to avoiding unstubbing
$wgUser to check $wgUser->isAllowed( 'read' ) in the common case where
'read' permission is granted to everyone.

For the OAuth work, we need to be able to catch that last type of use
without interfering with the others. This change introduces
User::isEveryoneAllowed() to be used for that type of check, which both
makes sure the right granted to '*' isn't revoked from any group and
calls a hook to allow extensions to indicate that they might remove the
right.

Change-Id: Idfee1b4d0613aaf52e143164acd6022459415c49