Fix new group url error.

Should I also make an issue or does this PR suffice?

This fixes the following error:

```
ActionController::UrlGenerationError in Groups#new

Showing /Users/stephan/development/shared/apps/onebody/app/views/groups/_form_advanced.haml where line #27 raised:

No route matches {:action=>"show", :controller=>"groups", :format=>nil, :"group[hidden]"=>true, :id=><>} missing required keys: [:id]
```