🎨 Optimized loading stripe scripts only when it is needed (#11499)

closes #11463

- Ghost used to always load stripe.js into the frontend of all pages when memberships are enabled, even when Stripe isn't configured / memberships to a page are free. This changes Ghost's behaviour to only load stripe.js when both stripe API tokens are present & not empty (the quickest way to verify that Stripe is fully configured & operational on a blog).
- Needs a follow-up cleanup removing confusing/not functional `isPaymentConfigured` method from members service

core/frontend/helpers/ghost_head.js

@@ -44,10 +44,17 @@ function finaliseStructuredData(metaData) {
 }
 
 function getMembersHelper() {
-    return `
-        <script src="https://js.stripe.com/v3/"></script>
-        <script defer src="${getAssetUrl('public/members.js')}"></script>
-    `;
+    const stripePaymentProcessor = settingsCache.get('members_subscription_settings').paymentProcessors.find(
+        paymentProcessor => paymentProcessor.adapter === 'stripe'
+    );
+    const stripeSecretToken = stripePaymentProcessor.config.secret_token;
+    const stripePublicToken = stripePaymentProcessor.config.public_token;
+
+    var membersHelper = `<script defer src="${getAssetUrl('public/members.js')}"></script>`;
+    if (!!stripeSecretToken && stripeSecretToken !== '' && !!stripePublicToken && stripePublicToken !== '') {
+        membersHelper += '<script src="https://js.stripe.com/v3/"></script>';
+    }
+    return membersHelper;
 }
 
 /**