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.