Add Redux and use it for state management

app/javascript/reducers/rootReducer.ts

@@ -0,0 +1,12 @@
+import { combineReducers } from 'redux';
+
+import postsReducer from './postsReducer';
+import postStatusesReducer from './postStatusesReducer';
+
+const rootReducer = combineReducers({
+  posts: postsReducer,
+  postStatuses: postStatusesReducer,
+});
+
+export type State = ReturnType<typeof rootReducer>
+export default rootReducer;