Add following UI in collab titlebar (#3544)

* [x] restructure titlebar to show facepiles for each collaborator and
their followers
* [x] allow following collaborators by clicking their avatars in the
titlebar
* [x] show grayscale avatar for collaborators not focused on this
project
* [x] show collaborators' microphone activity and muted status in the
titlebar
* [x] in facepile, show leader in front of followers