Passing complex objects to django-q async_task results in a
PicklingError during serialization which prevents report creation.
This change avoids the problem by only passing module-level functions
and regular Python data structures to async_task and between the
asynchronous function and the hook.

Without this property, the async task would use the default timeout, which is currently too short at 90 seconds.

This reverts commit aa3acb4.

Trying out whether the change was at all needed

This is just in case, because async email reports appear to sometimes
get eternally stuck in the queues if the worker is killed by a timeout.