Include GitHub's stylesheet.css in Storybook head

[vdepizzol]

A couple of Primer CSS components need GitHub's stylesheet.css to render correctly (I'm looking at you, dropdown). This adds it to the Storybook config directory along with preexisting stylesheet.js.

This fixes this:

So it renders as this:

/cc @primer/ds-core

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[jonrohan]

I think this was a bug, but there might be a reason. @shawnbot?

[shawnbot]

This doesn't appear to interfere with Storybook's styles since it's only rendered in previews. I think this works!

[shawnbot]

@vdepizzol I set the base branch for this to #854, but something just occurred to me:

This probably won't work on primer.github.io/storybook because of github.com's CORS headers, which will prevent other domains from loading them in a browser. We might need to revive the mechanism that pulled down a static version of the github.com asset(s) to get this working, but for now it at least makes things easier for local development.

[vdepizzol]

@shawnbot that makes sense! I didn't know primer.github.io/storybook existed :)