[SCHEMA] Alpha Yahoo mail sync support.

Summary:
Current caveats:
* No threading support. (Each message is a new thread.)
* No flag synchronization support.
* No actions support.
* No sending/drafts reconcilation.

Read-only mail synchronization totally works though.

Test Plan: Full sync of test account + made sure Gmail still works.

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D212