We heard about a bug where learners granted extensions would still
lose access to content if it was marked as "hidden after due date".
This was caused by the HiddenContentTransformer using the due date
from collection time (publish time) rather than the user date returned
from the edx-when DateOverrideTransformer.
A small subtletly of this PR is that Transformers with the
FilteringTransformerMixin are executed before those without it so
part of the fix was to make the HiddenContentTransformer not use the
FilteringTransformerMixin to ensure the DateOverrideTransformer had
run first.
Part 3/3 - Removing old collect code with merged due date
c4a96a2fa3