Calculate seconds left from timestamp

Previously seconds_left was sent from Rails and decremented in a
callback set to run every 1000ms, but of course this can lead to some
wildly inaccurate times.
Don't send seconds_left in Round json

It's no longer used anywhere
Redisplay time left every 100ms

Anything closer to one second can cause issues where the time appears to
change non-uniformily due to callback falling just before/after the end
of a second.
Update todo
update TODO
Merge branch 'recalculate_time_left'