Added sidebar menu feature

app/javascript/gabsocial/reducers/sidebar.js

@@ -0,0 +1,12 @@
+import { SIDEBAR_OPEN, SIDEBAR_CLOSE } from '../actions/sidebar';
+
+export default function sidebar(state={}, action) {
+  switch(action.type) {
+  case SIDEBAR_OPEN:
+    return { sidebarOpen: true };
+  case SIDEBAR_CLOSE:
+    return { sidebarOpen: false };
+  default:
+    return state;
+  }
+};