SecurityUserSettingsTab: Put the event index settings behind the feature flag.

src/components/views/settings/tabs/user/SecurityUserSettingsTab.js

@@ -254,12 +254,15 @@ export default class SecurityUserSettingsTab extends React.Component {
             </div>
         );
 
-        const eventIndex = (
+        let eventIndex;
+        if (SettingsStore.isFeatureEnabled("feature_event_indexing")) {
+            eventIndex = (
                 <div className="mx_SettingsTab_section">
                     <span className="mx_SettingsTab_subheading">{_t("Message search")}</span>
                     <EventIndexPanel />
                 </div>
             );
+        }
 
         // XXX: There's no such panel in the current cross-signing designs, but
         // it's useful to have for testing the feature. If there's no interest