Rework the user_groups system, again, into something that seems to actually

Rework the user_groups system, again, into something that seems to actually
more or less work for now.

* user_groups ur_group is now a short string key ('sysop' etc)
* groups table is gone
* user_rights table is gone
* Permissions for groups are for now set in $wgGroupPermissions.
An in-database management system could be re-added in the future
if it's really needed, but for now it's mostly just been screwing
things up.
* Group.php and Special:Groups are deprecated; will probably die.
* User group memberships are set explicitly through addGroup and
removeGroup methods instead of being re-saved on every change to
the user record.

Group keys are migrated from user_rights at upgrade time for older wikis.
The fields in prior 1.5alpha tables were too screwed up and will need to
manually have sysops re-assigned.

The Makesysop extension will need some minor tweaks.