Add Ushahidi_Repository abstract class for basic queries, fixes T265

Summary: - implements user repo using base class

Test Plan:
- login
- register a new account

Reviewers: dkobia, rjmackay, eyedol

Reviewed By: rjmackay

Subscribers: PapaLee, lkamau

Maniphest Tasks: T265

Differential Revision: https://phabricator.ushahidi.com/D113