Add "Features" configuration to site settings page, refs T172

Summary:
- [php] add `window.config.features`
- [php] add `config.format.hash` service for translating a config group to a plain hash
- [php] add default features config file
- [js] add features checkboxes to settings page
- [js] refactor saving settings for multiple groups
- [scss] add settings module for settings page, minor restyle

Test Plan:
- view settings page
- change site settings
- toggle features

Reviewers: seth, rjmackay

Reviewed By: seth, rjmackay

Subscribers: dkobia

Maniphest Tasks: T172

Differential Revision: https://phabricator.ushahidi.com/D46