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