Add closed attribute to group

closes #273 

ermageeeeeeeerd, I worked on the app!
please check pplz. 
Sometimes browsing through the other code in a project brings some inspiration. For example there is a `person_signed_in?` helper method that could be used here, instead of checking for nil. I saw you have changed the text in the spec already from 'is nil' to 'is not signed in', very nice!

I found the behaviour of the join buttons a bit confusing before this change already: they are there when not signed in, but hidden when signed in and the person is already a member. Maybe it has to be that way. But shouldn't the full-ness of a group always hide the join button, regardless of sign-in status? Otherwise, I might see the join button as a non-signed in user, then sign in only to see the button disappear - it would make me think the group does not want me specifically.

I suggest to add the full / not full variant to the groups_helper_spec as another context and specify what should happen in each case. Once that is safe, you can experiment with refactoring the boolean statement in the helper if you want.