Membership period_type is required in frontend but not in API.

When creating a new membership type in the UI, [fixed | rolling] is required. Naturally the API for MembershipType create should enforce this also.

In v4 can we query a command to see what the required fields are? (if this isn't a thing already....I don't see it though.)

Seems like then we could then make a test that could compare the form validation against our api requirements to make sure they always agree.