This PR allows Studio to record desktop audio and resolves elan-ev#551

Unfortunately, not all browsers are supported yet, as you can see here: https://caniuse.com/mdn-api_mediadevices_getdisplaymedia_audio_capture_support and as Lukas mentioned here: [elan-ev#551](elan-ev#551)

The users get a hint after selecting display as source, with which browsers they can record desktop audio.

Output of the recordings for download:

Only camera recorded: Nothing changed, the finished camera video will contain microphone audio (if recorded)

Only display recorded: If the user recorded both, desktop audio and microphone audio, the finished video will have both audio tracks.

Display and camera recorded: The finished desktop video will contain desktop audio and the finished camera video will contain the microphone audio

One thing/problem:
In the "Review & trim" step (also after uploading to Opencast), if both the display AND the camera were recorded: you only hear the microphone audio or nothing (depending on whether microphone audio was recorded or not).
I'm not entirely sure why.
The audio is correct and audible for each of the videos that can be downloaded in the last step in Studio.

With this, Studio pre-mixes all the audio tracks
from all the recordings, i.e. desktop and mic,
pre-mixes them and uses the resulting single audio track
in the videos that are uploaded to Opencast,
and those the user can download.

This also works in the review step.