Add multichannel audio support for player page

[kerrydp]

The existing Broadcast Box player page will let you listen to audio tracks one and two by default. However, when using OBS, I can stream six mono tracks (or three stereo pairs) on a WHIP stream.

It would be great to have an option to select which audio track you are listening to in the player window. This could either be six checkboxes for enabling/disabling the six mono tracks, or a drop-down to select one of three stereo pairs.