move days_ago to utils and make it tz aware

it's used in a few places - it should definitely know what timezones
are and return datetimes rather than dates, which are hard to work with
in terms of figuring out how tz aware they are.

app/dao/__init__.py

@@ -28,8 +28,3 @@ class DAOClass(object):
         db.session.delete(inst)
         if _commit:
             db.session.commit()
-
-
-def days_ago(number_of_days):
-    from datetime import date, timedelta
-    return date.today() - timedelta(days=number_of_days)