Add Optional CalendarReference to StudentSchoolAssociation

Summary of outcomes for Data Standard 3.0

• Problem Addressed: In v2.1, a Calendar reference was added to Cohort to allow a calendar to be assigned to a cohort of students. However, it's known that Cohort is not widely used.
• High-level Reasoning: By adding the Calendar reference to StudentSchoolAssociation instead, students may (optionally) be assigned a calendar for each school enrollment. The presence of a Calendar reference on StudentSchoolAssociation implies that this calendar should override the default school calendar for the student. This modeling approach is parallel to how staff are assigned a calendar.
• Changes Made: The Calendar reference was removed from Cohort and added to StudentSchoolAssociation. Cohort was removed from the SchoolCalendar domain and replaced by Student and StudentSchoolAssociation.

Original ticket description follows:
Associating a student with a calendar allows for attendance calculations.  The other information needed is already available (enrollment dates, absences, class periods scheduled).