Merge branch '212560-sse-app-skeleton' into 'master'

Create static site editor Vue app skeleton

See merge request gitlab-org/gitlab!28347

app/assets/javascripts/pages/static_site_editor/index.js

+import initStaticSiteEditor from '~/static_site_editor';
+
+window.addEventListener('DOMContentLoaded', () => {
+  initStaticSiteEditor(document.querySelector('#static-site-editor'));
+});

app/assets/javascripts/static_site_editor/components/static_site_editor.vue

+<template>
+  <div></div>
+</template>

app/assets/javascripts/static_site_editor/index.js

+import Vue from 'vue';
+import StaticSiteEditor from './components/static_site_editor.vue';
+import createStore from './store';
+
+const initStaticSiteEditor = el => {
+  const store = createStore();
+
+  return new Vue({
+    el,
+    store,
+    components: {
+      StaticSiteEditor,
+    },
+    render(createElement) {
+      return createElement('static-site-editor', StaticSiteEditor);
+    },
+  });
+};
+
+export default initStaticSiteEditor;

app/assets/javascripts/static_site_editor/store/index.js

+import Vuex from 'vuex';
+import Vue from 'vue';
+import createState from './state';
+
+Vue.use(Vuex);
+
+const createStore = ({ initialState } = {}) => {
+  return new Vuex.Store({
+    state: createState(initialState),
+  });
+};
+
+export default createStore;

app/assets/javascripts/static_site_editor/store/state.js

+const createState = (initialState = {}) => ({
+  content: '',
+  ...initialState,
+});
+
+export default createState;