refactor CreatesMembership to expose create_customer_and_membership
migration to allow nil stripe_* values in Customer

Necessary to support free memberships
MembershipPlan.free
User.is_member? and User.with_membership_plan
ActiveAdmin.register Membership

Gives us a route that redirects Membership.id to User.id
users can be created via the admin interface

And users can be given free memberships.
/admin/dashboard summarises users by membership plan type
users created by the admin interface are notified