Alerts checked on publish of a timetable and on publish of DO.
Example 1: email sent to a DP if a class becomes too small or too large, assessed for a particular date range
Example 2: teacher is absent for a particular class on more than 5 occasions in a particular date range
Example 3: The number of periods taken by emergency teachers is equal to or greater than x over the last x days.
User will need to be able to name the alert, create a message that is sent if triggered and set a date range