Convert cross-signing feature flag to setting

This is intended as a temporary measure until we're comfortable with removing the flag entirely.

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

@@ -270,7 +270,7 @@ export default class SecurityUserSettingsTab extends React.Component {
         // can remove this.
         const CrossSigningPanel = sdk.getComponent('views.settings.CrossSigningPanel');
         let crossSigning;
-        if (SettingsStore.isFeatureEnabled("feature_cross_signing")) {
+        if (SettingsStore.getValue("feature_cross_signing")) {
             crossSigning = (
                 <div className='mx_SettingsTab_section'>
                     <span className="mx_SettingsTab_subheading">{_t("Cross-signing")}</span>