luci-app-statistics: adjust graph size fallback to defaults

Adjust the fallback timespan and graph width to the current config
defaults. (The default width in /etc/config/luci_statistics has been
600 already since commit 7ab8b51b

 in March 2010.)

Expose the graph height in the config file and
increase it from 100 to 150 to keep the original aspect ratio.

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