Really fix the timezones

Two main changes:

- uses `astimezone` instead of `replace` because `replace` doesn’t
  handle daylight savings time correctly [1]
- create the notifications one second before midnight in BST, because
  midnight is actually counted as being start of the _next_ day, month,
  etc

1. http://www.saltycrane.com/blog/2009/05/converting-time-zones-datetime-objects-python/#add-timezone-localize

app/dao/notifications_dao.py

@@ -359,6 +359,6 @@ def get_april_fools(year):
 
 
 def get_bst_month(datetime):
-    return pytz.utc.localize(datetime).replace(
-        tzinfo=pytz.timezone("Europe/London")
+    return pytz.utc.localize(datetime).astimezone(
+        pytz.timezone("Europe/London")
     ).strftime('%B')