The products in an order may require several shipping categories like
refrigeration or express. We now filter the categories according to
their support of shipping categories.
This makes a clear distinction between the unfiltered and filtered list.
There may also be some gotchas when modifying the array of an
ActiveRecord relation. It also allows us to write shorter code without
storing a separate variable.
Before if a shipping method was shared between multiple distributors it could only be disabled/enabled on that order cycle for all the distributors which have that shipping method e.g. you couldn't select that shipping method for one distributor but disable it for another.
