If HELPDESK_ENABLE_PER_QUEUE_STAFF_MEMBERSHIP was True but a user had
no QueueMembership entries, then restricting queue access generated
RelatedObjectDoesNotExist exceptions.
- Ask for forgiveness whenever we try to limit a queryset by the
queuemembership related object set.
- Since tests can now be run with the project's settings active,
rather than only with quicktest.py's settings, restore the initial
HELPDESK_ENABLE_PER_QUEUE_MEMBERSHIP value after having tested the
related functionality.
