Adding/extending support for several unhandled block types (GitBook Page refs,Hint,Anchor,Emoji node,inline-image,etc)

[I-Dont-Remember]

Dove in to just fix page links, ended up finding and touching up a variety of other things I ran into.

Changes

• Now able to pull Spaces from a user's personal, not just an organization space
• GitBook page links were undefined, now are connected to the actual page metadata and look like [Submit](/docs/requests/submit "Submit a Request")
• Hint blocks were undefined, now are rendered as block quote with emoji (see README for example).
• inline images in some cases were undefined
• anchor links were showing as undefined
• Emoji node - should fix UnhandledPromiseRejection "Unknown inline type: emoji" #5
• Added unit tests for the new functionality
• Script for downloading all images of a Space

QA

• Unit tests pass
• Files formatted with Prettier
• type checks pass
• No undefined showing when run on actual Spaces

Areas for potential improvement

I was moving fast and didn't have the time to hit every area I might have liked to.

• I believe the files lookup dict could be combined with the spaceContent data that I added to the markdown renderer
• Would be nice to be able to pass a prefix to GitBook links rendering, in case you want to keep the same structure at the new destination for your markdown but it sits under a different path.