Add a Program ParticipationStatus element to StudentProgramAssociation

There is not a great place to store information on student eligibility for a program (versus active participation). Adding a ParticipationStatus descriptor to the GeneralStudentProgramAssociation would fill this gap.

Recommended descriptor values:

• Referred
• Eligible
• Not Eligible
• Active in Program
• Refused

In addition, the definition for BeginDate on GeneralStudentProgramAssociation should be updated to be for the student's initial involvement in the program, not necessarily when services started.

Suggested definition for BeginDate:

"The earliest date a student is involved with the program. Typically this is the date the student becomes eligible for the program."