New user group

Hi all. We talked about this earlier today on IRC. The 'repoadmin' right for controlling Code Review configuration was (by default) assigned to stewards. This isn't really something that stewards do.

So I've created a new group called 'svnadmins' who can access repository config. Bureaucrats can add/remove the bit, since they're a small and trusted group.

(Really most people can ignore this because they won't need it and it's boring maintenance stuff. But thought I should post in case anyone sees it and has questions)