luci-app-statistics: increase default amount of data items in RRD

Increase the default number of data items in the RRD database
from 144 to 288. This leads to smaller summarising/averaging periods,
visible especially in the day & week graphs. The averaging intervals
will be: 30s, 5min, 35min, 2h35min, 1d6h30min

(Note: this change only applies in a live router if the RRD database is
empty. E.g after reboot or after emptying the RRD database dir.)

Reference to discussion at #4065

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