🎨 Reused attribution for embeddable signup forms on own site

fixes https://github.com/TryGhost/Team/issues/3473

When a signup happens via the embeddable signup form on your own site, we'll use the default
attribution, so it indistinguishable from a signup via Portal.

ghost/portal/src/utils/helpers.js

@@ -869,6 +869,7 @@ export const transformApiTiersData = ({tiers}) => {
 
 /**
  * Returns the member attribution URL history, which is stored in localStorage, if there is any.
+ * @warning If you make changes here, please also update the one in signup-form!
  * @returns {Object[]|undefined}
  */
 export function getUrlHistory() {

ghost/signup-form/src/utils/api.tsx

@@ -19,7 +19,7 @@ export const setupGhostApi = ({siteUrl}: {siteUrl: string}) => {
                 email,
                 emailType: 'signup',
                 labels,
-                urlHistory: getUrlHistory()
+                urlHistory: getUrlHistory({siteUrl})
             });
 
             const response = await fetch(url, {

ghost/signup-form/src/utils/helpers.tsx

@@ -13,7 +13,46 @@ export function isMinimal(options: SignupFormOptions): boolean {
     return !options.title;
 }
 
-export function getUrlHistory(): URLHistory {
+/**
+ * Get the URL history when the form is embedded on the site itself.
+ */
+export function getDefaultUrlHistory() {
+    const STORAGE_KEY = 'ghost-history';
+
+    try {
+        const historyString = localStorage.getItem(STORAGE_KEY);
+        if (historyString) {
+            const parsed = JSON.parse(historyString);
+
+            if (Array.isArray(parsed)) {
+                return parsed;
+            }
+        }
+    } catch (error) {
+        // Failed to access localStorage or something related to that.
+        // Log a warning, as this shouldn't happen on a modern browser.
+
+        /* eslint-disable no-console */
+        console.warn(`[Signup-Form] Failed to load member URL history:`, error);
+    }
+}
+
+export function getUrlHistory({siteUrl}: {siteUrl: string}): URLHistory {
+    // If we are embedded on the site itself, use the default attribution localStorage, just like Portal
+    try {
+        if (window.location.host === new URL(siteUrl).host) {
+            const history = getDefaultUrlHistory();
+            if (history) {
+                return history;
+            }
+        }
+    } catch (error) {
+        // Most likely an invalid siteUrl
+
+        /* eslint-disable no-console */
+        console.warn(`[Signup-Form] Failed to load member URL history:`, error);
+    }
+
     const history: URLHistory = [];
 
     // Href without query string