Support storing to GitHub Gist

[NullVoxPopuli]

• requires GitHub OAuth
  • I'm using Auth0 for this
• read from any gist (via URL)
• format determined by file extension
• if gist ends up containing more than one file, show a notice about this, and that only the first file can be used at the moment.
• store document in local storage since we don't want to post to github gist as fast as we update the URL
• switch "file" management to a xstate machine, since it's gotten complicated

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[changeset-bot]

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Latest commit: c235e8c

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