Refactor IDE index to support EE specific code

**Why?**

- The web terminal feature is EE specific and will need to inject
  some specific options / functions into certain components.

app/assets/javascripts/ide/index.js

@@ -8,9 +8,21 @@ import { convertPermissionToBoolean } from '../lib/utils/common_utils';
 
 Vue.use(Translate);
 
-export function initIde(el) {
+/**
+ * Initialize the IDE on the given element.
+ *
+ * @param {Element} el - The element that will contain the IDE.
+ * @param {Object} options - Extra options for the IDE (Used by EE).
+ * @param {(e:Element) => Object} options.extraInitialData -
+ *   Function that returns extra properties to seed initial data.
+ */
+export function initIde(el, options = {}) {
   if (!el) return null;
 
+  const {
+    extraInitialData = () => ({}),
+  } = options;
+
   return new Vue({
     el,
     store,
@@ -32,6 +44,7 @@ export function initIde(el) {
       });
       this.setInitialData({
         clientsidePreviewEnabled: convertPermissionToBoolean(el.dataset.clientsidePreviewEnabled),
+        ...extraInitialData(el),
       });
     },
     methods: {
@@ -52,3 +65,18 @@ export function resetServiceWorkersPublicPath() {
   const webpackAssetPath = `${relativeRootPath}/assets/webpack/`;
   __webpack_public_path__ = webpackAssetPath; // eslint-disable-line camelcase
 }
+
+/**
+ * Start the IDE.
+ *
+ * @param {Objects} options - Extra options for the IDE (Used by EE).
+ */
+export function startIde(options) {
+  document.addEventListener('DOMContentLoaded', () => {
+    const ideElement = document.getElementById('ide');
+    if (ideElement) {
+      resetServiceWorkersPublicPath();
+      initIde(ideElement, options);
+    }
+  });
+}

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

-import { initIde, resetServiceWorkersPublicPath } from '~/ide/index';
+import { startIde } from '~/ide/index';
 
-document.addEventListener('DOMContentLoaded', () => {
-  const ideElement = document.getElementById('ide');
-  if (ideElement) {
-    resetServiceWorkersPublicPath();
-    initIde(ideElement);
-  }
-});
+startIde();

ee/app/assets/javascripts/pages/ide/index.js

+import { startIde } from '~/ide/index';
+
+function extraInitialData() {
+  // This is empty now, but it will be used in: https://gitlab.com/gitlab-org/gitlab-ee/issues/5426
+  return {};
+}
+
+startIde({
+  extraInitialData,
+});