Fix classes reducer to use {} as default state. Add subjects reducer.

reducers/classes.js

@@ -1,5 +1,5 @@
 export default function classes(state) {
 	if (typeof state === "undefined")
-		return [];
+		return {};
 	return state;
 }

reducers/index.js

@@ -2,10 +2,12 @@ import { combineReducers } from "redux";
 
 import timetable from "./timetable";
 import classes from "./classes";
+import subjects from "./subjects";
 
 const rootReducer = combineReducers({
 	timetable,
-	classes
+	classes,
+	subjects,
 });
 
 export default rootReducer;

reducers/subjects.js

@@ -0,0 +1,5 @@
+export default function subjects(state) {
+	if (typeof state === "undefined")
+		return [];
+	return state;
+}