users endpoint crud + search refs D473

Summary:
Covers entire crud + search/single. Still need a bit of work on validation of update/delete and on authorizer for few cases
Currently passes most tests, one that it does not pass is due to some additional work needed on authorizer and me methods in controller.

Test Plan: bin/behat -c application/tests/behat.yml -f progress application/tests/features/api.users.feature

Reviewers: aMoniker, rjmackay, shadowhand

Reviewed By: shadowhand

Subscribers: rjmackay, srutto

Maniphest Tasks: T854

Differential Revision: https://phabricator.ushahidi.com/D481