Min cost frontend update

This should correct the js so recalculated subsidies aren't applied to all rows.
In the case where there is no set usage rate for a policy, it derives it from the master usage rate. Related to this, @jhanggi I'd like to go over the price policy lists builder with you https://github.com/tablexi/nucore-open/blob/master/app/controllers/price_policies_controller.rb#L147-156 so I can get a better understanding of the price_policy and price_group relationship.