CLC-3626 LTI-embedded bCourses course site mediacasts

https://jira.ets.berkeley.edu/jira/browse/CLC-3626

I doubt this will be rolled out to production bCourses as is in the next release. We may find cross-browser problems with the iframe-embedded video, or want to tailor the widget formatting a bit more. Also, I'd like to check with the bCourses team on their preferred way to add the app to a course site. The current code requires entering LTI secrets by hand in the browser, which seem onerous.

However, if the code looks OK, it makes sense to me to get it into the codebase earlier rather than later. That will give us more time to view & test it in context. Also, the mediacasts UX seems to be getting hammered fairly hard at the moment, & I'd like to avoid the merge conflict cost of a long-lived working branch. And the new code won't appear to the general public until we start explicitly adding it to production bCourses sites.