Add possibility to approve users in installer's ACL policy

Incited by the amount of users with open wikis complaining about lots of spam users, I thought the main problem is that users are not given the option in the installer to properly "close" their wiki, even if one of the option is wrongly called "Closed Wiki". I added a checkbox to re-define "registered users" as "approved users".

I implemented two different solutions:
1. either change the user group in the acl.auth.php from `user` to `member` (that's the solution you would get if you checked out 74850f2 only)
2. or change the defaultgroup in local.php to `guest` (this is the solution I changed to with d2ea6dc)