Admin interface: Cal-ITP Benefits staff users can edit existing configuration

[angela-tran]

Implementation:

• Add a data migration that gives the Cal-ITP group permission to change the models listed under Acceptance Criteria
• In the ModelAdmins for each model, implement get_readonly_fields so that fields that should be not be editable from users in the Cal-ITP group are rendered as read-only

Acceptance Criteria

• For EligibilityTypes, EligibilityVerifiers, AuthProviders, PaymentProcessors, and TransitAgencys, Cal-ITP Benefits staff users can edit fields on those existing objects if the field is listed as writable for Cal-ITP in the Benefits admin configuration spreadsheet

Additional context

Depends on #2199
Part of #2001