R1:ab2cbbd9f946

Add a "test message" action for contact numbers

Summary: Depends on D20024. See D20022. Put something in place temporarily until we build out validation at some point.

Test Plan: Sent myself a test message.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20025