Added homepage redirect for unsubscribe urls

refs https://github.com/TryGhost/Team/issues/1495

With multiple newsletters, members are allowed to manage their newsletter pref via email unsubscribe link with member uuid. Since Portal is now taking over handling unsubscribe for members, we don't need to keep the current `/unsubscribe` page as Portal can load the member's newsletter pref on site home page directly.
The redirect change is only enabled behind the `multipleNewslettersUI` flag as its in beta.

core/frontend/services/routing/controllers/unsubscribe.js

@@ -1,11 +1,33 @@
 const debug = require('@tryghost/debug')('services:routing:controllers:unsubscribe');
 const path = require('path');
+const url = require('url');
+
+const urlUtils = require('../../../../shared/url-utils');
 const megaService = require('../../../../server/services/mega');
 const renderer = require('../../rendering');
+const labs = require('../../../../shared/labs');
 
 module.exports = async function unsubscribeController(req, res) {
     debug('unsubscribeController');
 
+    if (labs.isSet('multipleNewslettersUI')) {
+        const {query} = url.parse(req.url, true);
+
+        if (!query || !query.uuid) {
+            res.writeHead(400);
+            return res.end('Email address not found.');
+        }
+
+        const redirectUrl = new URL(urlUtils.urlFor('home', true));
+        redirectUrl.searchParams.append('uuid', query.uuid);
+        if (query.newsletter) {
+            redirectUrl.searchParams.append('newsletter', query.newsletter);
+        }
+        redirectUrl.searchParams.append('action', 'unsubscribe');
+
+        return res.redirect(302, redirectUrl.href);
+    }
+
     let data = {};
 
     try {

test/e2e-frontend/members.test.js

@@ -9,6 +9,7 @@ const settingsCache = require('../../core/shared/settings-cache');
 const DomainEvents = require('@tryghost/domain-events');
 const {MemberPageViewEvent} = require('@tryghost/member-events');
 const models = require('../../core/server/models');
+const {mockManager} = require('../utils/e2e-framework');
 
 function assertContentIsPresent(res) {
     res.text.should.containEql('<h2 id="markdown">markdown</h2>');
@@ -129,6 +130,33 @@ describe('Front-end members behaviour', function () {
         });
     });
 
+    describe('Unsubscribe', function () {
+        beforeEach(function () {
+            mockManager.mockLabsEnabled('multipleNewslettersUI');
+        });
+
+        afterEach(function () {
+            mockManager.restore();
+        });
+
+        it('should redirect with uuid and action param', async function () {
+            await request.get('/unsubscribe/?uuid=XXX')
+                .expect(302)
+                .expect('Location', 'http://127.0.0.1:2369/?uuid=XXX&action=unsubscribe');
+        });
+
+        it('should pass through an optional newsletter param', async function () {
+            await request.get('/unsubscribe/?uuid=XXX&newsletter=YYY')
+                .expect(302)
+                .expect('Location', 'http://127.0.0.1:2369/?uuid=XXX&newsletter=YYY&action=unsubscribe');
+        });
+
+        it('should reject when missing a uuid', async function () {
+            await request.get('/unsubscribe/')
+                .expect(400);
+        });
+    });
+
     describe('Price data', function () {
         it('Can be used as a number, and with the price helper', async function () {
             // Check out test/utils/fixtures/themes/price-data-test-theme/index.hbs