[SCHEMA] Canonicalize email addresses when comparing them.

Summary:
Specifically, this currently means removing any periods from Gmail addresses.
We'll still return whatever form was originally stored. I.e., if you authed as
ben.bitdiddle@gmail.com, that's what we use to send, and return in `/n/` API
queries, etc.

Test Plan:
Unit tests updated; auth and sending behavior manually tested; ran a
sync without breakage.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D245