System preferences should be configurable

Mailman has a few preferences which serve as defaults for new list members and are defined as constants in mailman/core/constants.py. These are:

    acknowledge_posts = False
    hide_address = True
    receive_list_copy = True
    receive_own_postings = True
    delivery_mode = DeliveryMode.regular
    delivery_status = DeliveryStatus.enabled

Currently these defaults can be changed only by patching the code. They should be configurable.