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/EmptyPosts.jsx

@@ -0,0 +1,59 @@
+import React from 'react';
+import propTypes from 'prop-types';
+
+import { useDispatch, useSelector } from 'react-redux';
+
+import { injectIntl, intlShape } from '@edx/frontend-platform/i18n';
+
+import { useIsOnDesktop } from '../data/hooks';
+import { selectAreThreadsFiltered, selectPostThreadCount } from '../data/selectors';
+import messages from '../messages';
+import { messages as postMessages, showPostEditor } from '../posts';
+import EmptyPage from './EmptyPage';
+
+function EmptyPosts({ intl, subTitleMessage }) {
+  const dispatch = useDispatch();
+
+  const isFiltered = useSelector(selectAreThreadsFiltered);
+  const totalThreads = useSelector(selectPostThreadCount);
+  const isOnDesktop = useIsOnDesktop();
+
+  function addPost() {
+    return dispatch(showPostEditor());
+  }
+
+  let title = messages.noPostSelected;
+  let subTitle = null;
+  let action = null;
+  let actionText = null;
+  let fullWidth = false;
+
+  const isEmpty = [0, null].includes(totalThreads) && !isFiltered;
+
+  if (!(isOnDesktop || isEmpty)) {
+    return null;
+  } if (isEmpty) {
+    subTitle = subTitleMessage;
+    title = messages.emptyTitle;
+    action = addPost;
+    actionText = postMessages.addAPost;
+    fullWidth = true;
+  }
+
+  return (
+    <EmptyPage
+      title={intl.formatMessage(title)}
+      subTitle={subTitle ? intl.formatMessage(subTitle) : null}
+      action={action}
+      actionText={actionText ? intl.formatMessage(actionText) : null}
+      fullWidth={fullWidth}
+    />
+  );
+}
+
+EmptyPosts.propTypes = {
+  subTitleMessage: propTypes.string.isRequired,
+  intl: intlShape.isRequired,
+};
+
+export default injectIntl(EmptyPosts);