Allow setting a default scale type preference for the traffic graphs wid...

...get

I originally submitted this at https://redmine.pfsense.org/issues/2994
but it seems that is not used for commits now so I am adding it here.
It works great on my box, and I can't see these changes causing any
issues on other types.

I like to have Autoscale (follow) set for the traffic graphs widget and
hate having to click on Autoscale (up) to change it to follow every
time I enter the dashboard page.

This preference is written to the traffic graph widget section in
config.xml

Since nothing has changed in master for these files it is probably ok
there as well...