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

res/css/views/elements/_FacePile.scss

@@ -15,6 +15,9 @@ limitations under the License.
 */
 
 .mx_FacePile {
+    display: flex;
+    align-items: center;
+
     .mx_FacePile_faces {
         display: inline-flex;
         flex-direction: row-reverse;