luci-app-statistics: option for max/avg data when not rrasingle

Introduce option that enables the user to select max values
instead of averages for graphs if the user has disabled RRAsingle.

The option defaults to average values, which have been the default
in Luci statistics.

Remove 'optional' from RRASingle, as it is a key option for statistics.

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