Track last selected area or city.

Track last selected area or city.

Unless a form was just submitted, no default option is set in a select
box, so the first option in alphabetical order is used.  That can be
annoying, for instance, for users who mainly process data in one city
and find another city selected by default in all dropdowns.

On form submission we now track the selected city or area in a session
variable, and retrieve it when showing a form with no default option.