Change how TeacherCandidateFieldworkExperience is associated to staff/school

TeacherCandidateFieldworkExperience (and StaffFieldworkExperience) was built under the assumption that fieldwork placements would be associated to schools.  In Tk20 (for UTRGV and SHSU), fieldwork placements are associated to staff and then the staff is associated to a school (or multiple schools).  

A work around was implemented to remove SchoolReference from the primary key in the TeacherCandidateFieldworkExperience and make it unbounded but a longer term more permanent solution needs to be determined. 

Ask working group how fieldwork placements are structured outside of TK20.  Would it be most logical to have a fieldwork reference in a StaffTeacherCandidateAssociation or a new FieldworkTeacherCandidateAssociation and FieldworkStaffAssociation.  Consider the business logic that seems to be most commonly used and then restructure the model to capture the data accordingly.