Created User entity and repository interface, fixes T206

Summary:
Repo spec is incomplete, as it does not define methods for updating or deleting users.
But this is enough to start an implementation of storage.

Test Plan: soon

Reviewers: dkobia, rjmackay

Reviewed By: rjmackay

Subscribers: rjmackay

Maniphest Tasks: T206

Differential Revision: https://phabricator.ushahidi.com/D57