Allow Element Call to be started without audio / video interface for the sake of a screen-sharing only session

[fkwp]

No description provided.

[dbkr]

Risk here might be that this would imply joining a call with no transceivers, which probably isn't possible (?). I suggest that we just always negotiate a usermedia channel as we do now, but keep the media muted (muting video now releases the webcam anyway so it will be effectively the same as that).

[EnricoSchw]

Changed matrix-js-sdk to enable feature changes in EC. When this PR is merged we can build PR for EC
matrix-org/matrix-js-sdk#3162

[EnricoSchw]

For testing we need to enable the feature in the in config.json with:

{
  ...
  "features": {
      "feature_group_calls_without_video_and_audio": true
    }
}