Add Guns Free School Act descriptor values to Discipline

The suggestion is based on a review of current state implementation extensions.

Multiple states have added elements around requirements for the GFSA, specifically to do with expulsion for a GFSA event and the length of the expulsion (if less than the mandated one year).

Suggestion is to add the following values to the Discipline descriptor:

• Expulsion under Guns Free School Act
• Expulsion under Guns Free School Act with Services

I believe DisciplineActionLength, ActualDisciplineActionLength, and DisciplineActionLengthDifferenceReason can then cover data collection around the one year mandated length and any reason this may have changed.