Add is_discounted field on ShippingMethod

shipping/models:ShippingMethod was missing the is_discounted attribute that
exists in the de-facto superclass in shipping/base:ShippingMethod.

Also added a few comments to clarify how they relate to each other, and
tests to check if is_discounted is present on various instances of
ShippingMethods.
Merge branch 'tangentlabs-issue/527/shippingmethods'