feat: add frontend-plugin-framework slots (#545)

Add the following `frontend-plugin-framework` slots:
* `logo_slot`
* `desktop_main_menu_slot`
* `desktop_secondary_menu_slot`
* `mobile_main_menu_slot`
* `course_info_slot`
* `learning_help_slot`
* `desktop_logged_out_items_slot`
* `mobile_logged_out_items_slot`
* `mobile_user_menu_slot`
* `desktop_user_menu_slot`
* `learning_user_menu_slot`
* `learning_logged_out_items_slot`
* `desktop_header_slot`

src/desktop-header/DesktopLoggedOutItems.jsx

@@ -0,0 +1,24 @@
+import React from 'react';
+import PropTypes from 'prop-types';
+
+const DesktopLoggedOutItems = ({ items }) => items.map((item, i, arr) => (
+  <a
+    key={`${item.type}-${item.content}`}
+    className={i < arr.length - 1 ? 'btn mr-2 btn-link' : 'btn mr-2 btn-outline-primary'}
+    href={item.href}
+  >
+    {item.content}
+  </a>
+));
+
+export const desktopLoggedOutItemsDataShape = PropTypes.arrayOf(PropTypes.shape({
+  type: PropTypes.oneOf(['item', 'menu']),
+  href: PropTypes.string,
+  content: PropTypes.string,
+}));
+
+DesktopLoggedOutItems.propTypes = {
+  items: desktopLoggedOutItemsDataShape,
+};
+
+export default DesktopLoggedOutItems;