Cart with soft-deletion

[Matt-Yorkley]

What? Why?

Closes #5358

Adds various missing specs for variant soft-deletion issues and adjusts CartService logic so that if a variant is soft-deleted whilst it's in an active cart or is added to a cart, the user gets useful feedback (and no fatal error problems), the item is removed and acts as if it has gone out of stock.

What should we test?

Load a shop page, then delete a variant that's already shown on the page, then add that variant to the cart. It should show an out-of-stock modal and remove it from the cart.

Release notes

Fixed issues around adding deleted variants to cart.

[luisramos0]

This is very good!! 👏

[Matt-Yorkley]

Rebased 👍

[luisramos0]

Awesome 👌

[sauloperez]

I think in the long term we should soft-delete the line item upon soft-deletion of the variant, ala ON DELETE CASCADE only that it'll be an UPDATE statement in this case.

[filipefurtad0]

Awesome @Matt-Yorkley ,

Checked two scenarios here:

User: Loads /shopfront (variant x is visible)
Admin: Deletes variant x
User: Adds variant x to the cart (pressing the up arrow a couple of times)
-> the "Reduced stock available" modal kicks in 🎉 and the variant x becomes greyed-out.

I think this is the main aim of this PR, this is ready to go.

The other scenario I checked is:

User: Loads /shopfront (variant x is visible)
User: Adds variant x to the cart (pressing the up arrow a couple of times)
Admin: Deletes variant x
-> In this case, the modal is not shown.
User: pressing Cart or Edit your cart will lead to the situation described on issue #5239

While this could be improved it surely introduces a huge improvement. Should I open a separate issue for the second test-case?