Restructure form processing in FormModel.js and PostModel.js

Summary:
- Combine fieldset, schema and validation processing.
- Trying to simplify PostModel.fieldsets()
- Add FormModel.getAttribute()

Extracted from D33, but shouldn't be any dependencies (probably minor conflicts)

Test Plan: - Check that forms still render properly for editing posts

Reviewers: eyedol, shadowhand

Reviewed By: shadowhand

Differential Revision: https://phabricator.ushahidi.com/D35