WIP: add storybook

Usage

checkout branch

gdk run (needed for styles)
yarn install
yarn storybook
go to localhost:6006

What does this MR do?

Adds storybook for the MR Widget
useful for working with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12601

Are there points in the code the reviewer needs to double check?

TODO:

• move to external repo
• add to gdk
• license clarification
• try importing mockdata/components directly from spec files (need to define describe in webpack or somewhere)
• dedupe webpack config

Why was this MR needed?

Screenshots (if relevant)

states

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?