Indicate 'no sync' when not receiving play signal
Simplify facilitator for single activities

 - remove tab bar and code for it
 - replace "addActivity" / "removeActivity" with vanilla
   "set('activities', ...)", always replacing all activities with the
   current one.
 - remove "(un)mute for all" for video embeds.
 - take steps to use the "expected" behavior for video control: when
   synced, as an admin, seeking or pausing the YouTube player should
   control everyone.  As a non-admin, when synced, seeking/pausing
   sets "intendToSync" to false and allows the move.

Outstanding issues: when multiple admins are connected, seeking has
weird behavior.