Change "Conversations" back to "Direct messages" and add warning in web UI (#18289)

Partially reverts #18146

app/javascript/mastodon/components/status_action_bar.js

@@ -266,6 +266,7 @@ class StatusActionBar extends ImmutablePureComponent {
       menu.push({ text: intl.formatMessage(messages.redraft), action: this.handleRedraftClick });
     } else {
       menu.push({ text: intl.formatMessage(messages.mention, { name: account.get('username') }), action: this.handleMentionClick });
+      menu.push({ text: intl.formatMessage(messages.direct, { name: account.get('username') }), action: this.handleDirectClick });
       menu.push(null);
 
       if (relationship && relationship.get('muting')) {