Add Storybook for Vue components

What does this MR do?

Adds Storybook, and a story for todo_button.vue. Originally proposed in RFC gitlab-org/frontend/rfcs#84 (closed).

Notes

• GitLab CSS is compiled with Webpack
• Storybook-related code is isolated within the storybook/ directory (including package.json)
• When merged, the Storybook should deploy to Pages, available at https://gitlab-org.gitlab.io/gitlab/storybook/

How to test

Starting in the gitlab's root directory:

1. Change directory into storybook/

   $ cd storybook

2. Install dependencies:

   $ yarn install

   💡 Note: there is also a yarn storybook that can be run in gitlab's root directory

3. Navigate to http://localhost:6006/?path=/story/components-todo-button--default. You should see the todo_button.vue component in the Storybook 😊

Screenshots (strongly suggested)

storybook-demo