Display Product Variant consistently in cart, checkout, order receipt.

Currently there are 3 frontend areas where a product variant is displayed in 3 slightly different ways.
This PR centralizes display logic into one template and simplifies the other templates.

This is what this looks like everywhere now:
![sylius_variant](https://f.cloud.github.com/assets/624921/1451845/eac6b3d8-42ae-11e3-8668-90e6879a03f4.JPG)