Make it possible to disable anonymous access via App.oauth, refs T475

Summary: Step 1: if oauth_secret is empty, force user login to get access token.

Test Plan:
Edit `config/ushahidiui.php` and set `client_secret => ''` (empty string).
Clear any local anonymous token by removing it from localStorage using the browser console:

  localStorage.removeItem('Ushahidi:anonymous_access_token')`

Logout and refresh the home page, verify that nothing appears before redirecting to user login.

Reviewers: rjmackay, bodacea

Reviewed By: bodacea

Subscribers: srutto, eyedol, aMoniker

Maniphest Tasks: T475

Differential Revision: https://phabricator.ushahidi.com/D330