Enhance EligibilityType model with expiration-related field(s)

[thekaveman]

With the coming Low-income / CalFresh enrollment flow, we'll need a way to display certain information and apply logic in-code for discounts that expire, e.g.

• Does the discount / eligibility type expire at all?
• How long after enrollment does the discount expire?
• How long before expiration can a user re-enroll?

For CalFresh, this will be used in a number of places, e.g.

• In Include expiry_date with all CalFresh / Low income enrollments #1877, we'll calculate and send an expiration date with the group enrollment
• In Enrollment success: Add expiration date text #1919, we'll display the date of expiration
• In Create new Enrollment error state for users already enrolled, not expired yet / not within re-enrollment window #1921, we'll display the date of expiration and the date at which re-enrollment can occur

We need a relatively simple way to make these decisions without (ideally) having to hardcode this logic into the templates / views, and to make this more modular for future, non-CalFresh flows that expire (e.g. students, enhancements to Agency cards).

Acceptance criteria

The following fields are added to the EligibilityType model:

• supports_expiration: a BooleanField indicating if the EligibiltyType supports expiration, default to False
• expiration_days: a PositiveSmallIntegerField indicating how many days after enrollment the associated discount expires, allow null but requiring a value > 0 when supports_expiration == True; see Django docs on Cleaning and validating fields that depend on each other and Validating model instances
• expiration_reenrollment_days: a PositiveSmallIntegerField indicating how many days before expiration a user is allowed to re-enroll for the discount, allow null but requiring a value > 0 when supports_expiration == True; see Django docs on Cleaning and validating fields that depend on each other and Validating model instances