[CONFIG] Change config names for both install/web oauth flows.

Summary:
This patch introduces a way to support OAuth via both the Google
`installed` API app type, as well as the conventional web callback method.
It includes a stub Flask app to handle the URL redirection.

Any production system will certainly use web oauth, but this lets us keep
the previous flexibilty for quick development.

Test Plan: Try changing your OAuth client id/secret to the production values.

Reviewers: spang, kav-ya

Reviewed By: kav-ya

CC: emfree

Differential Revision: https://review.inboxapp.com/D39