Set a client secret for the default ushahidiui client, refs T258

Summary: New version of the OAuth server (bshaffer) does not allow an empty client secret.

Test Plan: Try to login.

Reviewers: brianherbert, eyedol, rjmackay

Subscribers: dkobia

Maniphest Tasks: T258

Differential Revision: https://phabricator.ushahidi.com/D126