Add placeholder for persons with disabled webcam in the video area

[cgruber-uos]

Problem
Switching cams on and off leads to a disappearing video in the right hand video section of BBB. The reaction of the other user is often "oh, she/he is off" / "has no connection" / left the conferende. But the person is still there but only a bit more "hidden" in the participants section.

Solution
Add a placeholder, e.g. a circle with initials "CG" / "BB" / ... in the video section so people know the person is still there but has only quit the cam.

[antobinary]

Duplicate of #12481 / #13844