Add &nomutepreview option and apply &volume to previews

[comex]

When streaming video, it was already possible to monitor input audio by showing video controls for the local video preview and pressing the unmute button. This PR adds two features to enhance this functionality:

• Add an option, &nomutepreview, to automatically unmute the preview.

  If passed in sender mode, the preview still starts muted on the device selection screen, but gets unmuted when you press "start". This is to avoid undesired results if the wrong audio device is preselected.

  The option can also be passed in director mode in order to monitor the director's own stream. In this case it takes effect as soon as the director enables their microphone/video, since there's no separate device selection screen.

• Apply the &volume setting to local video previews (again, in both sender and director mode).

  By itself, this does not unmute the preview, but determines what volume you get if you press the unmute button. This can also be combined with &nomutepreview.

Documentation is included, but I wasn't able to test the formatting (I don't know how to replicate the Markdown-to-HTML process used for the docs site).

Features not included:

• Monitoring a subset of input audio devices

• Sending monitor output to a non-default audio device

Still, this is good enough for me. My use case involves setting an Elgato HDMI capture device as the video/audio input in order to stream gameplay (directly through vdo.ninja, no OBS). I need to monitor audio in order to hear the game.

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[steveseguin]

Thank you for the contribution. I can't blindly accept code changes of course, I wish I could, so I'll need to find time to review things closely. If all goes well, I'll either merge these changes or push my own version of the features to github and to production.