Sync the mocked attributes with current backend API

Summary:
* API endpoint: api/v2/attributes
  * remove old mocked endpoints (forms/:id/attributes)
* use the data of the application/tests/behat.yml of the current backend

Test Plan:
* on the 'Create post' page (/posts/create) after selecting a form
  * check if the form settings of the mocked attributes resource are shown correctly

Reviewers: shadowhand

Reviewed By: shadowhand

Subscribers: aMoniker, rjmackay

Differential Revision: https://phabricator.ushahidi.com/D524