Make static assets available on path alias

[shawnbot]

This does a couple of things:

• Adds a custom HTTP server that solely sets the Next app's assetPrefix to the deployed host in the NOW_URL env var if it's being served on another domain. This is what makes it work at primer.style/components/ ✨
• Adds PNGs for the favicon and apple touch icon to static/assets/, and updates their paths in the document component.
• Introduces the @zeit/next-sass plugin to generate our custom CSS build and put it in the right place.
• Includes all the stuff in Streamline npm publish and Now deploy files #242 ✨

If development process was changed

Nothing has changed development-wise. You still run npm run dev locally, and Now runs npm build then npm start, which also starts the server. The Next dev option is set to true if NODE_ENV !== 'production', so there's no need to set NODE_ENV=development.

Merge checklist

• Changed base branch to release branch
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

[vercel]

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[shawnbot]

@emplums let's review this and merge to master rather than holding it up for a release.

[emplums]

Looks good! Feel free to merge