Replace Foundation Reveal JS with a body class change, fixes T216

Summary:
- disables reveal.js
- update reveal config (and DOM) to use more generic class names
- use ModalRegion to trigger show/hide of modal
- fix the "Go Back" button in the modal

Test Plan: Try the create post modal, user edit modal, etc.

Reviewers: rjmackay

Reviewed By: rjmackay

Subscribers: seth, dkobia

Maniphest Tasks: T216

Differential Revision: https://phabricator.ushahidi.com/D66