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/KeyBackupPanel.js

@@ -326,7 +326,7 @@ export default class KeyBackupPanel extends React.PureComponent {
                     </AccessibleButton>
                 </div>
             );
-            if (this.state.backupKeyStored && !SettingsStore.isFeatureEnabled("feature_cross_signing")) {
+            if (this.state.backupKeyStored && !SettingsStore.getValue("feature_cross_signing")) {
                 buttonRow = <p>⚠️ {_t(
                     "Backup key stored in secret storage, but this feature is not " +
                     "enabled on this session. Please enable cross-signing in Labs to " +