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.jsx

@@ -0,0 +1,71 @@
+import React from 'react';
+
+import { useDispatch, useSelector } from 'react-redux';
+import { useRouteMatch } from 'react-router';
+
+import { injectIntl, intlShape } from '@edx/frontend-platform/i18n';
+
+import { ALL_ROUTES } from '../../data/constants';
+import { useIsOnDesktop, useTotalTopicThreadCount } from '../data/hooks';
+import { selectTopicThreadCount } from '../data/selectors';
+import messages from '../messages';
+import { messages as postMessages, showPostEditor } from '../posts';
+import EmptyPage from './EmptyPage';
+
+function EmptyTopics({ intl }) {
+  const match = useRouteMatch(ALL_ROUTES);
+  const dispatch = useDispatch();
+
+  const hasGlobalThreads = useTotalTopicThreadCount() > 0;
+  const topicThreadCount = useSelector(selectTopicThreadCount(match.params.topicId));
+
+  function addPost() {
+    return dispatch(showPostEditor());
+  }
+
+  const isOnDesktop = useIsOnDesktop();
+
+  let title = messages.emptyTitle;
+  let fullWidth = false;
+  let subTitle;
+  let action;
+  let actionText;
+
+  if (!isOnDesktop) {
+    return null;
+  }
+
+  if (match.params.topicId) {
+    if (topicThreadCount > 0) {
+      title = messages.noPostSelected;
+    } else {
+      action = addPost;
+      actionText = postMessages.addAPost;
+      subTitle = messages.emptyTopic;
+      fullWidth = true;
+    }
+  } else if (hasGlobalThreads) {
+    title = messages.noTopicSelected;
+  } else {
+    action = addPost;
+    actionText = postMessages.addAPost;
+    subTitle = messages.emptyAllTopics;
+    fullWidth = true;
+  }
+
+  return (
+    <EmptyPage
+      title={intl.formatMessage(title)}
+      subTitle={subTitle ? intl.formatMessage(subTitle) : null}
+      action={action}
+      actionText={actionText ? intl.formatMessage(actionText) : null}
+      fullWidth={fullWidth}
+    />
+  );
+}
+
+EmptyTopics.propTypes = {
+  intl: intlShape.isRequired,
+};
+
+export default injectIntl(EmptyTopics);