* Add a feature where a webcam broadcaster may manually invite others on
chat to watch, even if normally the other person would not be allowed.
For example, it will bypass the mutual webcam requirement setting and
allow the invited user to watch even if their own camera is not on.
* The button appears in Profile Modals and in the overflow menu on the
MessageBox component.
* Move all WebRTC and webcam-related functionality from App.vue into an
external Vue mixin.
* Centralize the stand-alone functions videoIconClass and
videoButtonClass into the new WebRTC mixin, which is called by
WhoListRow and MessageBox.
