This should not be get_or_create(), but rather just `Plan.create()`.  If we hit this branch, then we know the admin user was at the add view, so they want to *create* a new Plan.  

Related: Matching Stripe's functionality creates a weird UX problem here:  admin users (and customers) will use `Plan.name` to differentiate plans.  If we make these non-unique, and modifiable (Stripe's way), then we have no way to alert the admin user that they are duplicating names.  (I guess we could override the `Form.clean_name()`, but that feels silly).  Maybe they should be unique, but editable, even through that's not how Stripe does it.  @dollydagr:  Any preference?