Add association to ApplicationGroup
Raise exception when embedded models aren't associated with an parent ApplicationGroup
Improve ApplicationGroup association name
Update attribute name
Add broker association
Create association: irs_group has_many hbx_enrollment_exemptions
Remove bad embedded enrollee association
Add embedded applicants
Add associations to Person, Policy, Broker.  Add embedded models for enrollment_exemptions and eligibility_determinations.
Add enrollment_exemption specs