history: add timezone support

[b1rger]

Saving an entity without explicitly setting a history date gives a warning:

/usr/local/lib/python3.11/site-packages/django/db/models/fields/__init__.py:1595: RuntimeWarning: DateTimeField VersionPerson.history_date received a naive datetime (2024-04-16 10:43:27.944208) while time zone support is active.

https://stackoverflow.com/questions/18622007/runtimewarning-datetimefield-received-a-naive-datetime says:

In places where you were using datetime.now, change it to timezone.now, and add from django.utils import timezone at the top