Update Config API to act on whole groups, instead of single keys, fixes T204

Summary:
- update namespace to place Entity and Repository together
- store Config group name in `@group` property
- add setGroup and getGroup methods to Config
- drop `config.format.hash` entirely
- update storage, routes, API, and JS to match backend changes

Test Plan:
- use settings page to modify config values
- run behat tests `application/tests/features/api.config.feature`

Reviewers: rjmackay

Reviewed By: rjmackay

Maniphest Tasks: T204

Differential Revision: https://phabricator.ushahidi.com/D54