Updated checkout flow to pre-fill member email

refs https://github.com/TryGhost/members.js/issues/10

- Takes advantage of change here - 24b83bdc49 - to pre-fill customer email in anonymous checkout flow
- Member cannot change their email in this checkout session, allowing consistency and correct linking post checkout

ghost/portal/src/components/ParentContainer.js

@@ -144,11 +144,11 @@ export default class ParentContainer extends React.Component {
                     page: 'magiclink'
                 });
             } else if (action === 'signup') {
-                const {plan} = data;
+                const {plan, email} = data;
                 if (plan.toLowerCase() === 'free') {
                     await this.GhostApi.member.sendMagicLink(data);
                 } else {
-                    await this.GhostApi.member.checkoutPlan({plan});
+                    await this.GhostApi.member.checkoutPlan({plan, email});
                 }
                 this.setState({
                     action: 'signup:success',

ghost/portal/src/utils/api.js

@@ -111,7 +111,7 @@ function setupGhostApi({adminUrl}) {
             });
         },
 
-        async checkoutPlan({plan, checkoutCancelUrl, checkoutSuccessUrl}) {
+        async checkoutPlan({plan, checkoutCancelUrl, checkoutSuccessUrl, email: customerEmail}) {
             const identity = await api.member.identity();
             const url = endpointFor({type: 'members', resource: 'create-stripe-checkout-session'});
 
@@ -125,7 +125,8 @@ function setupGhostApi({adminUrl}) {
                     plan: plan,
                     identity: identity,
                     successUrl: checkoutSuccessUrl,
-                    cancelUrl: checkoutCancelUrl
+                    cancelUrl: checkoutCancelUrl,
+                    customerEmail: customerEmail
                 })
             }).then(function (res) {
                 if (!res.ok) {