Improve checkout process

Fixes #88, fixes #89, fixes #91
Fix default address handling
Add roles to navbars
Better shopping cart header button
Better step navigation in checkout
Move shorthand version of the adddress to a snippet
Make summary match order details
Add missing redirect after POST in product details
Make summary template easier to understand

Also fix help_text for alias and make sure we highlight missing template
variables.
Improve profile address management
Drop unused form class
Improve admin
Drop unused import
Avoid binding cart to checkout object

We don't want to tempt users to access it directly and shoot themselves
in the foot.
Fix import
Template fixes
Merge pull request #93 from mirumee/new-checkout

Improve checkout process