timetext: Cache the parsed timestamp on the object.
Use live timestamps for individual updates.

In testing, this looked really ugly and repetitive. But the actual
frequency of updates from real humans is such that using these live
timestamps is (a) more informative and (b) readable. Yay.
Spread out refreshes over five minutes.

The last refresh-storm overwhelmed our load balancer. This should give
us a little more breathing room in the future.