[Speedgrader] Make audio feedback that can be exported

[mark_crane]

Problem statement:

Currently if you want to leave audio-only feedback in the comment box in Speedgrader, you need to turn off the camera. However, unlike video, you can't right-click and download the recording.

Proposed solution:

Let instructors choose between audio or video feedback. Make the audio recording so it can be exported as an .mp3 if needed.

User role(s):

instructor