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/EventTile.js

@@ -323,7 +323,7 @@ export default createReactClass({
 
         // If cross-signing is off, the old behaviour is to scream at the user
         // as if they've done something wrong, which they haven't
-        if (!SettingsStore.isFeatureEnabled("feature_cross_signing")) {
+        if (!SettingsStore.getValue("feature_cross_signing")) {
             this.setState({
                 verified: E2E_STATE.WARNING,
             }, this.props.onHeightChanged);