feat: add "complete" (sic) timezone support

at least for now, this is enough to let admin define the global
default timezone for app, and override system local timezone.

eventually should support per-user timezone..some day..

tests/views/test_settings.py

@@ -46,6 +46,26 @@ class TestAppInfoView(WebTestCase):
         view = self.make_view()
         context = view.configure_get_context()
 
+    def test_configure_check_timezone(self):
+        view = self.make_view()
+
+        # normal
+        with patch.object(self.request, "GET", new={"tzname": "America/Chicago"}):
+            result = view.configure_check_timezone()
+            self.assertFalse(result["invalid"])
+
+        # invalid
+        with patch.object(self.request, "GET", new={"tzname": "bad_name"}):
+            result = view.configure_check_timezone()
+            self.assertEqual(
+                result["invalid"], "'No time zone found with key bad_name'"
+            )
+
+        # missing input
+        with patch.object(self.request, "GET", new={}):
+            result = view.configure_check_timezone()
+            self.assertEqual(result["invalid"], "Must provide 'tzname' parameter.")
+
 
 class TestSettingView(WebTestCase):