[TRACKING] Model refactor: create a new EnrollmentFlow model

[thekaveman]

This model will combine most fields from the existing EligibilityType and EligibilityVerifier models.

Acceptance criteria

Mostly in order of how this should be done.

• Model refactor: rename eligibility fields on EligibilityVerifier #2238
• Model refactor: move IdG config to EligibilityVerifier #2237
• Model refactor: rename EligibilityVerifier to EnrollmentFlow #2282
• Model refactor: move EligibilityType fields to EnrollmentFlow #2283
• A new model exists called EnrollmentFlow with all fields as documented (and in the same order) in the refactor proposal tab of the spreadsheet
• Existing Django config data is migrated
• Docs are updated

Impact on the code

• Code search for eligibility in the repo
• Code search for verifier in the repo

Impact on existing data

Documented in the issues tracked above under Approach.

Impact on analytics

• EligibilityType.name is logged in analytics using the eligibility_types event property and user property -- this is the primary field used in chart filters and groupings
• EligibilityType.group_id is logged in analytics using the payment_group event property
• EligibilityVerifier.name is logged in analytics using the eligibility_verifier event property and user property
• Some of these changes will need to be addressed in this ticket as the corresponding fields will no longer exist
• Model refactor: EnrollmentFlow Amplitude updates #2248 is a follow-up

Impact on Admin permissions

Documented in the issues tracked above under Approach.