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
Remove trailing whitespace.
Add a label for the `undefined` issues

Auditors: eater
Add explicit error for unknown problem type in dev

Otherwise you get a more-cryptic error later...

Auditors: emily
Fix drag behavior of 3D volume models in Firefox
Merge pull request #66227 from theinternetftw/volume-drag-fix

Fix drag behavior of 3D volume models in Firefox
Additional Firefox 3D model drag behavior fix
Merge pull request #66246 from theinternetftw/volume-drag-fix-2

Additional Firefox 3D model drag behavior fix