Fix false warnings

[andrewlimaza]

• BUG FIX: Fixes a bug where products that can only be purchased one per cart would incorrectly not allowed to be purchased even if it was the only membership product in the cart.

Hooking into wp allows us to use the is_cart and is_checkout functions from WooCommerce to avoid this error. Admins/Shop Owners should also limit purchases for products that should only be purchased one per cart within the "inventory" settings of WooCommerce.

For some reason the 'is_purchasable' was running multiple times on the checkout and cart page due to our updated code.

• Have you followed the Contributing guidelines?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?