Create a setting for which template is used when rendering an invoice as a PDF.
Revert "Create a setting for which template is used when rendering an invoice as a PDF."

This reverts commit 5ef6db6f71e55d06fb19d3dcc7a3cc5dad4a9190.
Add a new template for rendering invoices as PDFs. Simply extends the normal order invoice template by default.
Change views which use forms to accept a form_name argument, which is a dotted path to the form class.
Merge
Rename pdf invoice template for consistency and allow it to be overridden as view arg.
Make view args for forms actual class objects, and deprecate the SHOP_CHECKOUT_FORM_CLASS setting since we'll use a consistent approach for it too.