Move `Vuex.Store` call out of module

Also add `namespaced: true` to module

app/assets/javascripts/groups/members/index.js

 import Vue from 'vue';
 import Vuex from 'vuex';
 import App from './components/app.vue';
-import { createStore } from '~/vuex_shared/modules/members';
+import membersModule from '~/vuex_shared/modules/members';
 import { convertObjectPropsToCamelCase } from '~/lib/utils/common_utils';
 
 export default el => {
@@ -13,10 +13,12 @@ export default el => {
 
   const { members, groupId, currentUserId } = el.dataset;
 
-  const store = createStore({
-    members: convertObjectPropsToCamelCase(JSON.parse(members), { deep: true }),
-    sourceId: parseInt(groupId, 10),
-    currentUserId: currentUserId ? parseInt(currentUserId, 10) : null,
+  const store = new Vuex.Store({
+    ...membersModule({
+      members: convertObjectPropsToCamelCase(JSON.parse(members), { deep: true }),
+      sourceId: parseInt(groupId, 10),
+      currentUserId: currentUserId ? parseInt(currentUserId, 10) : null,
+    }),
   });
 
   return new Vue({

app/assets/javascripts/vuex_shared/modules/members/index.js

-import Vuex from 'vuex';
 import createState from './state';
 
-export const createStore = initialState =>
-  new Vuex.Store({
-    state: createState(initialState),
-  });
+export default initialState => ({
+  namespaced: true,
+  state: createState(initialState),
+});