AA-124: Refactor enrollment alerts (#126)

- Place them only on the Outline page
- Support a few cases where enrollment isn't actually allowed

src/tab-page/LoadedTabPage.jsx

@@ -3,7 +3,6 @@ import PropTypes from 'prop-types';
 
 import { Header, CourseTabsNavigation } from '../course-header';
 import { useModel } from '../generic/model-store';
-import { useEnrollmentAlert } from '../alerts/enrollment-alert';
 import InstructorToolbar from '../instructor-toolbar';
 
 function LoadedTabPage({
@@ -12,8 +11,6 @@ function LoadedTabPage({
   courseId,
   unitId,
 }) {
-  useEnrollmentAlert(courseId);
-
   const {
     isStaff,
     number,

src/tab-page/TabPage.jsx

@@ -3,7 +3,6 @@ import PropTypes from 'prop-types';
 import { injectIntl, intlShape } from '@edx/frontend-platform/i18n';
 
 import { Header } from '../course-header';
-import { useLogistrationAlert } from '../alerts/logistration-alert';
 import PageLoading from '../generic/PageLoading';
 
 import messages from './messages';
@@ -14,8 +13,6 @@ function TabPage({
   courseStatus,
   ...passthroughProps
 }) {
-  useLogistrationAlert();
-
   if (courseStatus === 'loading') {
     return (
       <>