Introduce DisplayMoney class mixin to clean up display_* method model logic.

There are a lot of model-level methods named display_ that just wrap a
numeric in a Spree::Money object. Rather than taking up space and mental
overhead in the models, this class mixin allows for defining of those
desired display_methods in a tidier way (with some configuration), in an effort to make some of
these models smaller and easier to reason about.