luci-app-statistics: shorten delay at graph endpoint to 15s

luci-app-statistics: shorten delay at graph endpoint to 15s

Shorten the visible delay at the statistics graph endpoint
from 60 seconds to 15 seconds.

The 60s delay was recently added as a way to remove a possible
visible gap at the end of the hourly graph due to absence of
recent data. Default data collection interval is 30s, so that
60s guarantees that there is data upto to the end of the graph.

However, that 60s makes any recent activity to get displayed
really slowly, after a 60-89 second delay.

Shortening the gap to 15s, half of the default data collection
period, should balance things:
* Half of the time there may be a really narrow gap visible and
  half of the time there is no gap at all.
* The most recent displayed data point is from 15-44 seconds ago
  (instead of 60-89 seconds ago).

Neither 15 or 60 seconds makes any impact to the longer graphs.
To accommodate to possibly shorter timespans, and to avoid the
occasional wrong data series selected for longer period graphs
(see #4065) the endpoint delay might be relative to the timespan.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>