Add ApplicationReference to CandidateEducatorPreparationProviderAssociation

Adding an optional Application reference would allow pre-enrollment characteristics be directly tied to a candidate's participation in a program. 

UConn as an example, some people get their undergraduate degree/Teaching certification, then apply/enroll/complete the principal leadership program/certification, then apply/enroll/complete the superintendent's program/certification. UConn wants to know their place of employment and job role at the time of the entry into each program. Currently the best way to do that is to use the CandidateIndicator. But that type of information can be captured in the Application domain entity so if a specific Application could be tied to a candidate's enrollment in a given program, the CandidateIndicator would not have to be used.