To correct bugs in groups and categories.

As stated in my commit message:

Fixing a bug which allowed groups to be created having the same name as an automatic group, precipitating chaos, as well as neglect to determine whether a user has the permission to create a topic on a category (besides being able to create a post) in Guardian#can_create_topic?, causing the "Create Topic" button to appear even if a user cannot actually create a topic in that category but can reply to a topic therein.