Iterate video room designs in labs (#8499)
* Remove blank header from video room view frame
* Add video room option to space context menu
* Remove duplicate tooltips from face piles
* Factor RoomInfoLine out of SpaceRoomView
* Factor RoomPreviewCard out of SpaceRoomView
* Adapt RoomPreviewCard for video rooms
* "New video room" → "Video room"
* Add comment about unused cases in RoomPreviewCard
* Make widgets in video rooms mutable again to de-risk future upgrades
* Ensure that the video channel exists when mounting VideoRoomView
