Introduce an event for related videos being rendered.

Summary:
Since this is done asynchronously, it's useful for client code to know when
the UI is ready. This is immediately needed for a video efficacy experiment
that renders a guider, suggesting a user watch a related video.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3779