Remove AcademicSubject from CredentialField descriptor

Summary of outcomes for Data Standard 3.0

• Problem Addressed: The CredentialField descriptor contained a reference to the AcademicSubject descriptor, which is not permitted in v3.0.
• High-level Reasoning: In reality, the AcademicSubject should pertain to the Credential itself and not necessarily a CredentialField. By moving the AcademicSubject descriptor to the Credential entity as an optional collection and removing it from the CredentialField descriptor, users can specify multiple subjects for which the credential applies.
• Changes Made: Remove AcademicSubject descriptor reference from CredentialField descriptor and add to Credential entity as an optional collection.

Original ticket description follows:

Only one descriptor remains in 3.0 that does not follow the consistent descriptor pattern. CredentialField descriptor contains a reference to AcademicSubject, which is acting like a map type. Map types were removed in 3.0, so it's likely that AcademicSubject should be removed from CredentialField.