Fix recording settings (define constraints)

[ericyd]

• Define constraints on audio when getting user media. This doesn't make sense why this is required, but it makes the recording actually work on my audio interface, whereas previously the audio would fade in and out inexplicably.
• Create the track's stream on track mount, rather than on Start. This makes properties a bit easier to pass around, and prevents re-using the same stream for multiple tracks. I'm not sure this is illegal, but this felt like a better approach