Add infrastructure for sending SMS via AWS SNS

Summary:
Ref T920. Ref T13235. This adds a Future, similar to TwilioFuture, for interacting with Amazon's SNS service.

Also updates the documentation.

Also makes the code consistent with the documentation by accepting a media argument.

Test Plan: Clicked the "send test message" button from the Settings UI.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T13235, T920

Differential Revision: https://secure.phabricator.com/D19982