#1491 - improve time zone handling

Handle the user's time zone from Canvas more carefully.  Don't assume it is set, doesn't have extraneous whitespace, or is valid.  If it's not set or invalid, use the `TIME_ZONE` set in `env.hjson`.

dashboard/lti_new.py

@@ -224,7 +224,11 @@ def extract_launch_variables_for_tool_use(request, message_launch):
     username = custom_params['user_username']
     course_id = custom_params['canvas_course_id']
     canvas_user_id = custom_params['canvas_user_id']
-    time_zone = custom_params['person_address_timezone']
+    time_zone = custom_params.get('person_address_timezone',
+                                  settings.TIME_ZONE).strip()
+
+    if time_zone not in pytz.all_timezones:
+        time_zone = settings.TIME_ZONE  # default zone from `env.hjson`
 
     canvas_course_long_id = canvas_id_to_incremented_id(course_id)
     canvas_user_long_id = canvas_id_to_incremented_id(canvas_user_id)