Improved group privacy settings

:question:  : **CR** 

:question: : **QA** 

In this commit we have

* renamed group.viewable_by = 'members' to group.privacy = 'secret' (and changed 'everyone' to 'public')
* removed the group privacy drop-down toggle on the privacy icon
* combined the group setup form and group edit form

So if you're QA this, you should make sure to test:

* adding subgroups
* making new groups
* updating existing groups
* changing privacy settings

### QA Checklist
> *test core functionality of feature:*
> :question: check front page loads

> :question: logged out functionality
> ...

> :question: test in IE 8,9 
> :question: test in production environment

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:question: : **CO signoff** 
:question: : **DO signoff**

:question: : **PO signoff**