Added Stripe form email to the blank shipping address form

As we have a customer email address filled in the Stripe form, let's re-use it in the "shipping address" form :-)
Paper books shipping process is almost finished :-)

Added an email sending on "shipping address" form submission
Added a specific message on "paper book show" page when user has bought it and submitted its address

If the user has bought a paper book and has submitted its address form, we no longer display a "buy" button on the book "show" page
Added the books sorting by "weight" when the "findAll()" method is used on the Books Repository