Implement checkout process

[edvinnn]

Implement checkout process.

• [x] View (buttion in cart --> confirm view --> redirect to detail order view (needs implementation)

• [x] Back-end (migrate cart_items to order_products)

• [ ] Write and design testing

[Venefilyn]

• [x] When ordering check if all products' quantity is enough
  • If it is, proceed with order and reduce quantity
  • If it is not, redirect back with message using django.contrib

[edvinnn]

Halvvägs genom detta... back-end kvar nu bara

[edvinnn]

Klar med implementation. Tester ej gjorda.