[events/participants] Provide initial support for participant RSVPs.

Summary:
We will need a way for participants to be able to indicate that they
are attending a particular event. This diff provides the API changes and tests
to provide such support. Issues such as access to this link need to be provided
through redwood white-listing the appropriate event endpoint.

Test Plan: Run tests which exercise success and failure cases of API.

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D366