The timezone improvements that went in only covered some of the cases
where timezones were set. Most weren't actually covered, as several
fields were using default=datetime.now.

This switches these to use django.utils.timezone.now instead. It also
switches away from djblets.util.dates.get_tz_aware_utcnow, which does
the same thing as Django's and will soon be removed.

Saw that all the warnings from unit tests went away.