Distinguish publication and updated date in feed items

[sunarch]

It would be useful if both published and updated dates could be added to feed items, not just the updated field.

I added the new field to the tests but wasn't able to run them properly because the current master produces FileNotFoundErrors on the feeds and that is beyond this scope.

Reference feedgenerator pubdate here: https://github.com/django/django/blob/stable/5.2.x/django/utils/feedgenerator.py#L472