Subscriptions Management: Line items with a global (cart) subscription scheme can't be switched

[manospsyx]

As discussed in #278, a limitation of the switching module in SATT 2.1 is that line items created by selecting a cart-level subscription scheme can't be switched.

Fixing this will require two things:

1. If a product doesn't have any product-level schemes, check if global (cart-level) schemes exist.
2. Allow switching.
3. When redirecting the user to the single-product page, use the global schemes to populate the product object.

(3) is likely the hardest part, as we'll need to somehow detect that we're in "switch" mode and (probably) filter the loaded product-level schemes.

[manospsyx]

Moved to the APFS repository.