Add form attribute entity and repository

Summary:
- Add FormAttribute Entity and Spec
- Add FormAttributeRepository
- Add Ushahidi_Repository_FormAttribute

Test Plan:
./bin/phpspec run spec/Ushahidi/Entity/FormAttributeSpec.php
No repositories tests yet

Reviewers: dkobia, shadowhand

Reviewed By: shadowhand

Differential Revision: https://phabricator.ushahidi.com/D223