Add a root index endpoint, fixes T1021

Summary:
- allow REST controllers to bypass oauth check
- fix base API URL generation when no `$id` is given
- add "index" route

Test Plan: Go to `/api/v2` in your local install.

Reviewers: vladimir, sheesh, aMoniker

Reviewed By: aMoniker

Subscribers: spaudanjo, rjmackay, srutto

Maniphest Tasks: T1021

Differential Revision: https://phabricator.ushahidi.com/D469