Start on Many to many form attributes #21

* I've made attribute key unique
* Moved /forms/:id/attributes/:id to /forms/attributes/:id
* Added join tables (though think maybe I only need the form_groups one)
  * forms_form_attributes
  * form_groups_form_attributes
* You can only create attributes with a form + group - since orphan attributes aren't very useful.
* Need to work out what the API call would be for adding an existing attribute to a form group..
* Not too many issues, but more joins required.
Many to Many form attributes #21

* Move /form/attributes to /attributes
* Update /form/:id/groups/:id/attributes
  * POST can now add existing attributes
  * DELETE remove attribute from the group, doesn't delete it permanently
  * GET /form/:id/groups/:id/attributes/:id now works
* Remove forms_form_attributes table
  join through form_groups_form_attributes instead
* Split group attributes api calls into seperate controll
* Split group attributes tests into seperate feature