add subs-specific validation on can_edit_order

[helgatheviking]

Core does a rather lot of validation, but we probably need to also check we can switch the sub... Ie. Settings allow it.

Still unsure how to handle switch between contents AND switch between variations... If you switch variations you most likely will have to switch contents too.

Maybe you have to have both switch between variations (subs core tho.. ditch our setting)

Switch between variations alone will do nothing on a variable MNM.
Switch between contents but not variations - switch variations like a simple product.. contents only.