Create new settings page

[MAILPOET-2676]
The new settings can be accessed via `?page=mailpoet-new-settings`
I don't think we need a flag for this because it's already hidden.

assets/js/src/old_settings/premium_tab/messages/premium_messages.jsx

@@ -1,7 +1,7 @@
 import PropTypes from 'prop-types';
 import React from 'react';
 import MailPoet from 'mailpoet';
-import { PremiumInstallationMessages } from 'settings/premium_tab/messages/premium_installation_messages.jsx';
+import { PremiumInstallationMessages } from 'old_settings/premium_tab/messages/premium_installation_messages.jsx';
 
 const PremiumStatus = {
   KEY_INVALID: 0,

assets/js/src/old_settings/premium_tab/premium_tab.jsx

@@ -2,10 +2,10 @@ import PropTypes from 'prop-types';
 import React, { useMemo, useState } from 'react';
 import ReactDOM from 'react-dom';
 import MailPoet from 'mailpoet';
-import KeyMessages from 'settings/premium_tab/messages/key_messages.jsx';
-import { MssStatus, MssMessages } from 'settings/premium_tab/messages/mss_messages.jsx';
-import { PremiumStatus, PremiumMessages } from 'settings/premium_tab/messages/premium_messages.jsx';
-import { PremiumInstallationStatus } from 'settings/premium_tab/messages/premium_installation_messages.jsx';
+import KeyMessages from 'old_settings/premium_tab/messages/key_messages.jsx';
+import { MssStatus, MssMessages } from 'old_settings/premium_tab/messages/mss_messages.jsx';
+import { PremiumStatus, PremiumMessages } from 'old_settings/premium_tab/messages/premium_messages.jsx';
+import { PremiumInstallationStatus } from 'old_settings/premium_tab/messages/premium_installation_messages.jsx';
 
 const requestServicesApi = async (key, action) => MailPoet.Ajax.post({
   api_version: window.mailpoet_api_version,

assets/js/src/old_settings/settings.jsx

@@ -1,6 +1,6 @@
 import React from 'react';
 import ReactDOM from 'react-dom';
-import DefaultSender from 'settings/default_sender.jsx';
+import DefaultSender from 'old_settings/default_sender.jsx';
 import { GlobalContext, useGlobalContextValue } from 'context/index.jsx';
 import Notices from 'notices/notices.jsx';
 

assets/js/src/settings/index.tsx

@@ -0,0 +1,9 @@
+import React from 'react';
+import ReactDOM from 'react-dom';
+
+const App = () => <h1 className="title">Settings</h1>;
+
+const container = document.getElementById('settings_container');
+if (container) {
+  ReactDOM.render(<App />, container);
+}

assets/js/src/webpack_admin_index.jsx

@@ -9,12 +9,13 @@ import 'subscribers/subscribers.jsx'; // side effect - renders ReactDOM to docum
 import 'newsletters/newsletters.jsx'; // side effect - renders ReactDOM to window
 import 'segments/segments.jsx'; // side effect - renders ReactDOM to document
 import 'dynamic_segments/dynamic_segments.jsx'; // side effect - renders ReactDOM to document
-import 'settings/settings.jsx'; // side effect - renders ReactDOM to document
+import 'old_settings/settings.jsx'; // side effect - renders ReactDOM to document
 import 'forms/forms.jsx'; // side effect - renders ReactDOM to document
-import 'settings/tabs.js'; // side effect - assigns to MailPoet.Router, executes code on doc ready
-import 'settings/premium_tab/premium_tab.jsx'; // side effect - renders ReactDOM to document
+import 'old_settings/tabs.js'; // side effect - assigns to MailPoet.Router, executes code on doc ready
+import 'old_settings/premium_tab/premium_tab.jsx'; // side effect - renders ReactDOM to document
 import 'help/help.jsx'; // side effect - renders ReactDOM to document
-import 'settings/reinstall_from_scratch.js'; // side effect - adds event handler to document
+import 'old_settings/reinstall_from_scratch.js'; // side effect - adds event handler to document
+import 'settings/index.tsx'; // side effect - renders Settings to document
 import 'subscribers/importExport/import.jsx'; // side effect - executes on doc ready, adds events
 import 'subscribers/importExport/export.js'; // side effect - executes on doc ready
 import 'wizard/wizard.jsx'; // side effect - renders ReactDOM to document