Add voice comments to annotations

### Problem you are trying to address with this feature
From Zendesk:
"I've been playing around a bit with Kaizena, an extension in Google Docs that allows a reader to leave voice comments on documents. Students really seem to dig these kinds of comments and they can be really helpful for students - - signaling a deeper engagement by the teacher, simulating a tutorial/conference, personalizing feedback, etc.

I wonder if there's a way that hypothesis can host voice comments?"

### Your solution
Allow for users to record short audio files in addition to / in place of a text-based annotation.

### User requests
- https://hypothesis.zendesk.com/agent/tickets/2235
- https://hypothesis.zendesk.com/agent/tickets/3081