Fix broken bits of api "config" endpoint, refs T172

Summary:
- add "features" group to database config
- clean up ConfigModel (js)

Test Plan:
- Use site settings page
- Make manual calls to API using D17

Reviewers: rjmackay

Reviewed By: rjmackay

Maniphest Tasks: T172

Differential Revision: https://phabricator.ushahidi.com/D31