feat: drop timezone, assume UTC for all datetime values in DB

i guess it was worth a try, but preserving system timezone was doomed
to failure since only postgres actually supports it.  from now on all
DateTime columns will be naive, but understood / assumed to be
UTC-local

tests/test_app.py

@@ -571,7 +571,7 @@ app_title = WuttaTest
             now = datetime.datetime.now()
             result = self.app.render_time_ago(now)
             self.assertEqual(result, "now")
-            humanize.naturaltime.assert_called_once_with(now)
+            humanize.naturaltime.assert_called_once()
 
     def test_get_person(self):
         people = self.app.get_people_handler()