Wrong submission archive urls when timezones are enabled

[jkuczm]

With enabled timezone support (USE_TZ = True in settings), Submission.get_absolute_url returns url with date in UTC but SubmissionArchiveDetailView (which inherits from DateDetailView which inherits from DateMixin) returns Submission instance based on publish_date in current time zone.

It can happen that link from archive page <newsletter_slug:s>/archive/ leads to non existing url, with day later or earlier by one.

In answer to ticket 18794 using DateField instead of DateTimeField or overriding get_object is suggested.

This is also related to documentation of DateMixin and documentation of timezones troubleshooting (item 3.).

[dokterbob]

First: implement test demonstrating this issue.

Then: fix issue according to suggested solution.