Ensure deleting a user doesn't delete their orders too

This change ensures that the order model's FKs all use
on_delete=SET_NULL.
Allow customers to delete their profile

We add a new form to the profile section that requires the customer to
confirm their password, but which then deletes their user.
Ensure templates are compatible with Django 1.4
Add migration for order on_delete changes
Merge pull request #1142 from tangentlabs/issue/1142/delete-profile

Customers should be able to delete their accounts