fixes #1818 video glitches when clicking quiz thumbnail

Candy: adds time labels in each thumbnail.

The glitch is fixed by using a "flip-flop".  When a displayQuestion
event fires, the event disables itself from firing again.  A paired
event .5 second later re-enables the event.  This prevents the
"knife-edge" effect which was causing the repeated looping.
1s -> 0.5s
slightly taller slidestrip
small nitty fix
Merge pull request #1823 from Stanford-Online/jbau-video-glitch

Jbau video glitch