Add visual tests with Storybook & Chromatic

[mattxwang]

This PR:

• sets up the basic framework for Storybook
• adds two UI component tests for buttons and labels (intentionally picked trivial examples)
• adds action to publish chromatic on push
• (out of code) sets up a Chromatic account for just the docs, which is synced to all GitHub collaborators

Draft for now as I'm tinkering around; major blockers are:

• importing "raw" HTML files with relative paths from _site
• dealing with theme changes
• JS-based functionality

Related to #805.

[mattxwang]

@pdmosses at your convenience, do you want to take a look at this PR? You should have access to our Chromatic account through GitHub OAuth; you should be able to deny/approve the change I made in 3c816ce (changing the color of a purple button).

If you think this UI/service supports the use-case you've described better than Percy, I can then work on adding more components, etc. and getting this merged in. On top of the simple cases I've added, we can also load some JS (ex for search/copy code), add some interactive workflows, etc. - chromatic's website has a few product use-cases (which are mostly free for us to use).

[pdmosses]

@pdmosses at your convenience, do you want to take a look at this PR?

I'm afraid I'm unlikely to have much time at all for JTD this month, and I need to prioritise submitting a PR to fix #1323, so I'll have to defer looking at this.

[mattxwang]

No worries at all; this isn't urgent!