Cleaned up members isPaid flag in settings table (#11651)

no issue

- The flag has not been used and can be removed, to make the `members_subscription_settings` JSON record in `settings` table easier to read. 
- It used to indicate Stripe configuration being present. Currently that is checked by looking up if Stripe config's `public_token` and `secret_token` values are present (example - https://github.com/TryGhost/Ghost/blob/3.11.0/core/frontend/helpers/ghost_head.js#L54)

core/server/data/migrations/versions/3.12/01-remove-legacy-is-paid-flag-from-settings.js

@@ -0,0 +1,93 @@
+const common = require('../../../../lib/common');
+const debug = require('ghost-ignition').debug('migrations');
+
+module.exports.config = {
+    transaction: true
+};
+
+module.exports.up = (options) => {
+    const settingsKey = 'members_subscription_settings';
+
+    return options
+        .transacting('settings')
+        .where('key', settingsKey)
+        .select('value')
+        .first()
+        .then((subscriptionSettingsEntry) => {
+            debug(subscriptionSettingsEntry);
+            if (!subscriptionSettingsEntry) {
+                common.logging.warn(`Cannot find ${settingsKey} settings.`);
+                return;
+            }
+
+            let subscriptionSettings = JSON.parse(subscriptionSettingsEntry.value);
+
+            debug('before cleanup');
+            debug(JSON.stringify(subscriptionSettings, null, 2));
+
+            const hasIsPaid = Object.prototype.hasOwnProperty.call(subscriptionSettings, 'isPaid');
+
+            if (hasIsPaid) {
+                debug('Removing legacy isPaid flag from members settings');
+                delete subscriptionSettings.isPaid;
+
+                debug('after cleanup');
+                debug(JSON.stringify(subscriptionSettings, null, 2));
+
+                return options
+                    .transacting('settings')
+                    .where('key', settingsKey)
+                    .update({
+                        value: JSON.stringify(subscriptionSettings)
+                    });
+            }
+        });
+};
+
+module.exports.down = (options) => {
+    const settingsKey = 'members_subscription_settings';
+
+    return options
+        .transacting('settings')
+        .where('key', settingsKey)
+        .select('value')
+        .first()
+        .then((subscriptionSettingsEntry) => {
+            debug(subscriptionSettingsEntry);
+            if (!subscriptionSettingsEntry) {
+                common.logging.warn(`Cannot find ${settingsKey} settings.`);
+                return;
+            }
+
+            let subscriptionSettings = JSON.parse(subscriptionSettingsEntry.value);
+
+            debug('before cleanup');
+            debug(JSON.stringify(subscriptionSettings, null, 2));
+
+            let isPaid = false;
+
+            const stripePaymentProcessor = subscriptionSettings.paymentProcessors.find(
+                paymentProcessor => paymentProcessor.adapter === 'stripe'
+            );
+
+            if (stripePaymentProcessor && stripePaymentProcessor.config.public_token && stripePaymentProcessor.config.secret_token) {
+                isPaid = true;
+            }
+
+            subscriptionSettings.isPaid = isPaid;
+
+            debug('after cleanup');
+            debug(JSON.stringify(subscriptionSettings, null, 2));
+
+            return options
+                .transacting('settings')
+                .where('key', settingsKey)
+                .update({
+                    value: JSON.stringify(subscriptionSettings)
+                });
+        });
+};
+
+module.exports.config = {
+    transaction: true
+};

core/server/data/schema/default-settings.json

@@ -207,7 +207,7 @@
             "defaultValue": "public"
         },
         "members_subscription_settings": {
-            "defaultValue": "{\"isPaid\":false,\"fromAddress\":\"noreply\",\"allowSelfSignup\":true,\"paymentProcessors\":[{\"adapter\":\"stripe\",\"config\":{\"secret_token\":\"\",\"public_token\":\"\",\"product\":{\"name\":\"Ghost Subscription\"},\"plans\":[{\"name\":\"Monthly\",\"currency\":\"usd\",\"interval\":\"month\",\"amount\":\"\"},{\"name\":\"Yearly\",\"currency\":\"usd\",\"interval\":\"year\",\"amount\":\"\"}]}}]}"
+            "defaultValue": "{\"fromAddress\":\"noreply\",\"allowSelfSignup\":true,\"paymentProcessors\":[{\"adapter\":\"stripe\",\"config\":{\"secret_token\":\"\",\"public_token\":\"\",\"product\":{\"name\":\"Ghost Subscription\"},\"plans\":[{\"name\":\"Monthly\",\"currency\":\"usd\",\"interval\":\"month\",\"amount\":\"\"},{\"name\":\"Yearly\",\"currency\":\"usd\",\"interval\":\"year\",\"amount\":\"\"}]}}]}"
         }
     },
     "bulk_email": {