Refactor model to split eligibility from benefit

[thekaveman]

The EligibilityType model contains two properties used by different phases of the system:

• EligibilityType.name is used in the eligibility phase as part of the API call to verify
• EligibilityType.group_id is used in the enrollment phase as the group to enroll eligible users into

However, as noted and fixed in #89, we may have multiple EligiblityType with the same name and different group_id. For example, a senior type for each transit agency, each with a different group_id.

The fix in #89 was somewhat of a hack, as these concerns could (should?) be split out to allow reusing EligibilityType instances. Thinking of a refactor along the lines of:

Before

class EligibilityType(models.Model):
    """Currently used for both eligibility and enrollment"""
    name = models.TextField()
    label = models.TextField()
    group_id = models.TextField()

class TransitAgency(models.Model):
   # other fields ...
    eligibility_types = models.ManyToManyField(EligibilityType)

After

class EligibilityType(models.Model):
    """Used for eligibility checks - reused across agencies"""
    name = models.TextField()
    label = models.TextField()

class BenefitGroup(models.Model):
    """Used for enrollment - at least one per agency"""
    name = models.TextField()
    group_id = models.TextField()
    eligibility_type = models.ForeignKey(EligibilityType, on_delete=models.PROTECT)

class TransitAgency(models.Model):
   # other fields ...
   benefit_groups = models.ManyToManyField(BenefitGroup)

[thekaveman]

Closing for now. This data model / relationship hasn't been a blocker, and the issue is creating noise on the backlog. Revisit in the future if needed.