WIP Add redirect URLs for order items.

@wedaly @rlucioni  Here's the initial idea. If this works for you guys I"ll write some tests. 
My theory was that this is view-level display logic, and if the template wants to know what to do with a donation, it should check it's order items to determine if one is a donation. 

This particular logic and context addition is now in the view, but if that doesn't sound reasonable to you, we could revisit that as well.