Admin: Cal-ITP Benefits staff user can add EnrollmentFlow models

[angela-tran]

This is for breaking up the part of #2638 that lets staff members add EnrollmentFlow models.

Acceptance Criteria

• Cal-ITP staff member can add a new EnrollmentFlow:
  • can add an EnrollmentFlow with claims verification to a transit agency
  • can start creation of an EnrollmentFlow with API verification; they have to ask an engineer to finish out configuration of the Eligibility API fields
• Cal-ITP staff member should not be able to add incomplete EnrollmentFlow to a transit agency