Added precaution to avoid creating multiple Complimentary plans

refs https://github.com/TryGhost/Ghost-Admin/pull/1430

- When the client creates a complimentary plan with other currency than USD we should not allow for it to avoid creating a mess in the Stripe plans

ghost/members-api/lib/stripe/index.js

@@ -36,6 +36,11 @@ module.exports = class StripePaymentProcessor {
 
             this._plans = [];
             for (const planSpec of config.plans) {
+                // NOTE: we have only one "Complimentary" plan throughout the system
+                if (planSpec.name === 'Complimentary' && planSpec.currency !== 'usd') {
+                    return;
+                }
+
                 const plan = await api.plans.ensure(this._stripe, planSpec, this._product);
                 this._plans.push(plan);
             }