Add Storybook for Dagit components

Summary:
Introduce Storybook so that we have a playground for component testing and iteration, add some TopNav examples.

Also modify TopNav to occupy a little less vertical space.

Test Plan:
yarn storybook within dagit, verify that it renders nicely.

Run dagit, verify that top nav renders as expected.

Reviewers: bengotow, alangenfeld, sashank, prha

Reviewed By: sashank

Differential Revision: https://dagster.phacility.com/D4685