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/rooms/RoomHeader.js

@@ -168,7 +168,7 @@ export default createReactClass({
         const joinRule = joinRules && joinRules.getContent().join_rule;
         let privateIcon;
         // Don't show an invite-only icon for DMs. Users know they're invite-only.
-        if (!dmUserId && SettingsStore.isFeatureEnabled("feature_cross_signing")) {
+        if (!dmUserId && SettingsStore.getValue("feature_cross_signing")) {
             if (joinRule == "invite") {
                 privateIcon = <InviteOnlyIcon />;
             }