Changes on MeetingTime and ClassPeriod to support BellSchedule and Intervention

Consider the following design for Bell Schedule:

The use of MeetingTime changes to be just the StartTime and EndTime as PK, then Period is changed of name to specify just the period (to use it in Intervention) and it's a specialization of MeetingTime since there is specified the name of the Period and if that period is used for OfficialAttendancePeriod, besides this model is more flexible since the use of two PeriodName with different StartTime and EndTime.

For Intervention we also have a MeetingTime but in this case the Period is optional.