Add Storybook for Dagit components
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.
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