feat!: organize plugin slots as components, add footer slot (#1381)

BREAKING CHANGE: slot ids have been changed for consistency
* `sequence_container_plugin` -> `sequence_container_slot`
* `unit_title_plugin` -> `unit_title_slot`

src/generic/CourseAccessErrorPage.jsx

@@ -1,9 +1,9 @@
 import React, { useEffect } from 'react';
 import { LearningHeader as Header } from '@edx/frontend-component-header';
-import Footer from '@edx/frontend-component-footer';
 import { useParams, Navigate } from 'react-router-dom';
 import { useDispatch, useSelector } from 'react-redux';
 import { injectIntl, intlShape } from '@edx/frontend-platform/i18n';
+import FooterSlot from '@openedx/frontend-slot-footer';
 import useActiveEnterpriseAlert from '../alerts/active-enteprise-alert';
 import { AlertList } from './user-messages';
 import { fetchDiscussionTab } from '../course-home/data/thunks';
@@ -32,7 +32,7 @@ const CourseAccessErrorPage = ({ intl }) => {
         <PageLoading
           srMessage={intl.formatMessage(messages.loading)}
         />
-        <Footer />
+        <FooterSlot />
       </>
     );
   }
@@ -51,7 +51,7 @@ const CourseAccessErrorPage = ({ intl }) => {
           }}
         />
       </main>
-      <Footer />
+      <FooterSlot />
     </>
   );
 };