Warn user if they attempt to select a video without a transcript

If an instructor attempts to create a Canvas Studio assignment using a video that doesn't have a transcript, it will fail with an error when launched. It would be a better user experience if we showed a warning earlier when they created the assignment, instead of waiting until they launch it.

A complication here is that we use a different API token, belonging to an admin user, to fetch transcripts compared to listing media. Therefore it is possible that listing media works fine but checking the transcript availability fails.