Activate the cart system when disconnected

[antho1404]

When a user is not connected, we hide the button to add to the cart while the checkout button stays here.
We should have a similar experience for checkout or the cart system. A few things we can do:

• Keep the button but disable it
• Keep the button and on click ask to login
• Keep the button and manage cart for not-connected users

Ideally, the last option would be the best.

Originally posted by @antho1404 in #546 (comment)