Add dismissable hints to various timelines in web UI (#19315)

Co-authored-by: Yamagishi Kazutoshi <ykzts@desire.sh>

app/javascript/mastodon/features/explore/links.js

@@ -6,6 +6,7 @@ import LoadingIndicator from 'mastodon/components/loading_indicator';
 import { connect } from 'react-redux';
 import { fetchTrendingLinks } from 'mastodon/actions/trends';
 import { FormattedMessage } from 'react-intl';
+import DismissableBanner from 'mastodon/components/dismissable_banner';
 
 const mapStateToProps = state => ({
   links: state.getIn(['trends', 'links', 'items']),
@@ -29,9 +30,17 @@ class Links extends React.PureComponent {
   render () {
     const { isLoading, links } = this.props;
 
+    const banner = (
+      <DismissableBanner id='explore/links'>
+        <FormattedMessage id='dismissable_banner.explore_links' defaultMessage='These news stories are being talked about by people on this and other servers of the decentralized network right now.' />
+      </DismissableBanner>
+    );
+
     if (!isLoading && links.isEmpty()) {
       return (
         <div className='explore__links scrollable scrollable--flex'>
+          {banner}
+
           <div className='empty-column-indicator'>
             <FormattedMessage id='empty_column.explore_statuses' defaultMessage='Nothing is trending right now. Check back later!' />
           </div>
@@ -41,6 +50,8 @@ class Links extends React.PureComponent {
 
     return (
       <div className='explore__links'>
+        {banner}
+
         {isLoading ? (<LoadingIndicator />) : links.map(link => (
           <Story
             key={link.get('id')}