🔨 Load bootstrap in storybook

Since Bootstrap is loaded in the admin too for Formio, we must include
it in storybook for an accurate preview.

Note that while a CDN is used in the admin, to facilitate local/offline
development, we're using the NPM package here.

.storybook/preview.js

@@ -1,3 +1,4 @@
+import 'bootstrap/dist/css/bootstrap.css';
 import '../src/openforms/scss/screen.scss';
 import '../src/openforms/scss/admin/admin_overrides.scss';
 import {withModalDecorator, withReactSelectDecorator, TinyMceDecorator} from 'components/admin/form_design/story-decorators';

package.json

@@ -100,6 +100,7 @@
     "autoprefixer": "^10.2.4",
     "babel-loader": "^8.2.2",
     "babel-plugin-formatjs": "^10.5.1",
+    "bootstrap": "~4.6.0",
     "browserslist": "^4.21.5",
     "copy-webpack-plugin": "^6.4.1",
     "css-has-pseudo": "^6.0.1",

src/openforms/js/components/admin/form_design/StepsFieldSet.stories.js

@@ -9,7 +9,15 @@ import {StepsFieldSet} from './form-creation-form';
 
 export default {
   title: 'Form design / Tabs / Steps',
-  decorators: [FormDecorator, AdminChangeFormDecorator],
+  decorators: [
+    FormDecorator,
+    Story => (
+      <div className="react-form-create">
+        <Story />
+      </div>
+    ),
+    AdminChangeFormDecorator,
+  ],
   component: StepsFieldSet,
   args: {
     submitting: false,