Add support for rate-limiting the sending API.

Summary:
Limit each account to a quota of 300 sent messages (by default) in a rolling 24
hour window. The quota value can be changed via the DEFAULT_SENDING_LIMIT
config value.

Fixes T449.

Test Plan: Unit test added.

Reviewers: spang, kav-ya

Reviewed By: kav-ya

Maniphest Tasks: T449

Differential Revision: https://review.inboxapp.com/D796