Add bin/inbox-consistency-check debugging tool for gmail accounts

Summary:
There are still some differences that haven't been ironed out, and some stuff
isn't implemented (e.g. labels, tags, flags), but the major pieces are here,
and they should already be useful.

This also serves as an example of how to use setuptools entry_points to
make plugins.

Test Plan: manual testing

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D507