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 = (
-            <div className="mx_SettingsTab_section">
-                <span className="mx_SettingsTab_subheading">{_t("Message search")}</span>
-                <EventIndexPanel />
-            </div>
-        );
+        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