Add backend support for saving a transcript selection with a YouTube assignment

When an instructor creates a YouTube assignment in the LMS app the frontend will in future be able to show the user a list of the video's available transcripts and lets the user select which transcript they want to use for the assignment.

1. We need to decide how the frontend will communicate this choice of assignment to the backend, when creating an assignment. For example perhaps a transcript ID in a query param. Note that not all assignment configurations are stored in our own DB: Canvas ones are stored in a launch URL in Canvas itself.
2. We need to decide how the backend will store this choice of transcript in its DB. Note again that Canvas assignment configurations are stored in Canvas not in our own DB, so we'll need to handle that.
3. When launching an assignment with a transcript selection present LMS should communicate the selected transcript to Via, see: https://github.com/hypothesis/via/issues/1057