Github "project" for getting Mediacapture-Main to PR

I set up a new project for this repo:

https://github.com/w3c/mediacapture-main/projects/1

All the procedure we used (or didn't use) in WebRTC-pc applies:

https://github.com/w3c/webrtc-pc/blob/master/PROJECT.md

As a starting point, I copied all open issues into the "To do" column and
put a few issues into the "In progress" column - things I know we've had
quite a bit of discussion on, or which are labelled "PR exists".

Feel free to move stuff around according to the procedure.

Harald