Used plans trial by default for checkout sessions (#158)

no-issue

Without this flag the checkout session will ignore any default trial periods
attached to the plan. Now we are able to give basic support for trials, by
attaching a trial period in Stripe Dashboard

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

@@ -124,6 +124,7 @@ module.exports = class StripePaymentProcessor {
             customer_email: customerEmail,
             metadata,
             subscription_data: {
+                trial_from_plan: true,
                 items: [{
                     plan: plan.id
                 }]