refactor; separate app.js to 3 files (except step yet)

src/Mine/SeekerBundle/Resources/public/js/mine-seeker/game/step.js

@@ -0,0 +1,10 @@
+import React from 'react';
+
+class MineStep extends React.Component {
+    constructor() {
+        super();
+    }
+
+}
+
+export default MineStep;