Second pass at cleaning up config class extension mess, refs T172

Summary:
- recreate Kohana_Config_Database the way it was supposed to be
- refactor Ushahidi_Config_Database based on new Kohana_Config_Database
- clean up api/config endpoint again, small changes for clarity

Test Plan:
- use site settings tools
- check XHR logs

Reviewers: rjmackay

Reviewed By: rjmackay

Subscribers: lkamau, eyedol

Maniphest Tasks: T185, T172

Differential Revision: https://phabricator.ushahidi.com/D38