feat: show empty state on posts pages [BD-38] [TNL-9484] [SE-5347] (#65)

When a user browses the discussion pages, show a page showing
that there is no data, rather than a blank page.

This PR implements the designs for this page while refactoring the
DiscussionHome component so that it doesn't get much larger.

- Created a hooks file and move some of the larger hooks in there.
- Added selectors for the state that would be needed for the
  components.
- Split the DiscussionHome into DiscussionContent and
  DiscussionSidebar to make it clearer where certain things get
  rendered.
- Adds a message to the sidebar to display No Results when
  appropriate.
- Added the NoResults component to show when filtering posts and there
  is nothing to show.

src/discussions/empty-posts/EmptyTopics.test.jsx

@@ -0,0 +1,77 @@
+import { render, screen } from '@testing-library/react';
+import MockAdapter from 'axios-mock-adapter';
+import { IntlProvider } from 'react-intl';
+import { Context as ResponsiveContext } from 'react-responsive';
+import { MemoryRouter } from 'react-router';
+import { Factory } from 'rosie';
+
+import { initializeMockApp } from '@edx/frontend-platform';
+import { getAuthenticatedHttpClient } from '@edx/frontend-platform/auth';
+import { AppProvider } from '@edx/frontend-platform/react';
+
+import { API_BASE_URL } from '../../data/constants';
+import { initializeStore } from '../../store';
+import { executeThunk } from '../../test-utils';
+import messages from '../messages';
+import { fetchCourseTopics } from '../topics/data/thunks';
+import EmptyTopics from './EmptyTopics';
+
+import '../topics/data/__factories__';
+
+let store;
+const courseId = 'course-v1:edX+DemoX+Demo_Course';
+const topicsApiUrl = `${API_BASE_URL}/api/discussion/v1/course_topics/${courseId}`;
+
+function renderComponent(location = `/${courseId}/topics/`) {
+  return render(
+    <IntlProvider locale="en">
+      <ResponsiveContext.Provider value={{ width: 1280 }}>
+        <AppProvider store={store}>
+          <MemoryRouter initialEntries={[location]}>
+            <EmptyTopics />
+          </MemoryRouter>
+        </AppProvider>
+      </ResponsiveContext.Provider>
+    </IntlProvider>,
+  );
+}
+
+async function setupMockResponse() {
+  const axiosMock = new MockAdapter(getAuthenticatedHttpClient());
+  axiosMock
+    .onGet(topicsApiUrl)
+    .reply(200, {
+      courseware_topics: Factory.buildList('category', 2),
+      non_courseware_topics: Factory.buildList('topic.withThreads', 3, {}, { topicPrefix: 'ncw' }),
+    });
+  await executeThunk(fetchCourseTopics(courseId), store.dispatch, store.getState);
+}
+
+describe('EmptyTopics', () => {
+  beforeEach(async () => {
+    initializeMockApp({
+      authenticatedUser: {
+        userId: 3,
+        username: 'abc123',
+        administrator: true,
+        roles: [],
+      },
+    });
+
+    store = initializeStore({ config: { provider: 'legacy' } });
+  });
+
+  test('"no topic selected" text shown when viewing topics page', async () => {
+    renderComponent(`/${courseId}/topics/`);
+    expect(screen.queryByText(messages.emptyTitle.defaultMessage))
+      .toBeInTheDocument();
+  });
+
+  test('"no post selected" text shown when viewing a specific topic', async () => {
+    await setupMockResponse();
+    renderComponent(`/${courseId}/topics/ncwtopic-3/`);
+
+    expect(screen.queryByText(messages.noPostSelected.defaultMessage))
+      .toBeInTheDocument();
+  });
+});