Ensure the purchased entity is available before adding to cart

Created patch without using availability manager and checking if Product Variation is published inside WishlistUserForm.

vmarchuk → made their first commit to this issue’s fork.

The last patch does not apply to the latest dev or beta.

Also, for a more consistent UX, we probably should disable the button, rather than hide it.

update to the MR incoming.

Re #13: I would expect for unavailable purchasable entities to be available to the wishlist, at least to some extent. If the item is not available, it means (at least in my head) it is not purchasable. But one might add it to the wish list to track whether it becomes available in the future.