Export mr_notes module as function

This is helpful at ensuring a fresh store between
specs.

app/assets/javascripts/mr_notes/stores/index.js

@@ -9,7 +9,7 @@ Vue.use(Vuex);
 export const createStore = () =>
   new Vuex.Store({
     modules: {
-      page: mrPageModule,
+      page: mrPageModule(),
       notes: notesModule(),
       diffs: diffsModule(),
     },

app/assets/javascripts/mr_notes/stores/modules/index.js

@@ -2,11 +2,11 @@ import actions from '../actions';
 import getters from '../getters';
 import mutations from '../mutations';
 
-export default {
+export default () => ({
   state: {
     activeTab: null,
   },
   actions,
   getters,
   mutations,
-};
+});

ee/app/assets/javascripts/mr_notes/stores/index.js

@@ -10,7 +10,7 @@ Vue.use(Vuex);
 export const createStore = () =>
   new Vuex.Store({
     modules: {
-      page: mrPageModule,
+      page: mrPageModule(),
       notes: notesModule(),
       diffs: diffsModule(),
       batchComments: batchCommentsModule(),