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
- 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.