Unified form JS handling for snippets

The unification of JS handling for the Project and Personal
snippets happened in abstracting the shared functionality
(pretty much all of it) into a shared file to have one SSOT for
4 (2 'edit' + 2 'new') views

app/assets/javascripts/pages/projects/snippets/edit/index.js

-import $ from 'jquery';
-import initSnippet from '~/snippet/snippet_bundle';
-import initForm from '~/pages/projects/init_form';
-
-document.addEventListener('DOMContentLoaded', () => {
-  initSnippet();
-  initForm($('.snippet-form'));
-});
+import '~/snippet/snippet_edit';

app/assets/javascripts/pages/projects/snippets/new/index.js

-import $ from 'jquery';
-import initSnippet from '~/snippet/snippet_bundle';
-import initForm from '~/pages/projects/init_form';
-
-document.addEventListener('DOMContentLoaded', () => {
-  initSnippet();
-  initForm($('.snippet-form'));
-});
+import '~/snippet/snippet_edit';

app/assets/javascripts/pages/snippets/edit/index.js

-import initSnippet from '~/snippet/snippet_bundle';
-import form from '../form';
-
-document.addEventListener('DOMContentLoaded', () => {
-  initSnippet();
-  form();
-});
+import '~/snippet/snippet_edit';

app/assets/javascripts/pages/snippets/new/index.js

-import initSnippet from '~/snippet/snippet_bundle';
-import form from '../form';
-
-document.addEventListener('DOMContentLoaded', () => {
-  initSnippet();
-  form();
-});
+import '~/snippet/snippet_edit';

app/assets/javascripts/snippet/snippet_edit.js

+import $ from 'jquery';
+import initSnippet from '~/snippet/snippet_bundle';
+import ZenMode from '~/zen_mode';
+import GLForm from '~/gl_form';
+
+document.addEventListener('DOMContentLoaded', () => {
+  const form = document.querySelector('.snippet-form');
+  const personalSnippetOptions = {
+    members: false,
+    issues: false,
+    mergeRequests: false,
+    epics: false,
+    milestones: false,
+    labels: false,
+    snippets: false,
+  };
+  const projectSnippetOptions = {};
+
+  const options =
+    form.dataset.snippetType === 'project' ? projectSnippetOptions : personalSnippetOptions;
+
+  initSnippet();
+  new ZenMode(); // eslint-disable-line no-new
+  new GLForm($(form), options); // eslint-disable-line no-new
+});

app/views/shared/snippets/_form.html.haml

@@ -3,7 +3,8 @@
 
 .snippet-form-holder
   = form_for @snippet, url: url,
-             html: { class: "snippet-form js-requires-input js-quick-submit common-note-form" } do |f|
+             html: { class: "snippet-form js-requires-input js-quick-submit common-note-form" },
+             data: { "snippet-type": @snippet.project_id ? 'project' : 'personal'} do |f|
     = form_errors(@snippet)
 
     .form-group