Added upgrade checkout metadata for stripe

no issue

Ghost sends email on successful signup to a paid plan via Stripe checkout, but couldn't differentiate between a Stripe checkout as new member or as an upgrade previously. This led to an extra second email with magic link on upgrading to paid plan while logged in.

Passing `checkoutType` metadata to Stripe checkout allows webhook to ignore second email in case of an upgrade scenario.

ghost/portal/src/actions.js

@@ -114,7 +114,10 @@ async function checkoutPlan({data, state, api}) {
     try {
         const {plan} = data;
         await api.member.checkoutPlan({
-            plan
+            plan,
+            metadata: {
+                checkoutType: 'upgrade'
+            }
         });
     } catch (e) {
         return {