Update data source tutorial to use more dependable API example

[jlengstorf]

It turns out the IMDB API is pretty unreliable, which is a bummer. This makes it a bad choice for an example data source.

We should revamp the tutorial to use a more reliable API. Requirements are:

• No API key required — we want people to immediately start coding, not have to sign up for API keys
• Reliable — should be up more often than not
• Relatively simple — if we can find a small API with one method, that would actually be the best option. A complete example that doesn’t involve a ton of copy-pasting would make the tutorial much more approachable than having to click through to GitHub for the rest of the code and/or copying hundreds of lines of code to get a proof-of-concept running

I'm currently leaning toward the xkcd API, which is small, reliable, and familiar to many developers.

[jlengstorf]

An example xkcd response:

{
  "month": "10",
  "num": 1905,
  "link": "",
  "year": "2017",
  "news": "",
  "safe_title": "Cast Iron Pan",
  "transcript": "",
  "alt": "If you want to evenly space them, it's easiest to alternate between the Arctic and Antarctic. Some people just go to the Arctic twice, near the equinoxes so the visits are almost 6 months apart, but it's not the same.",
  "img": "https://imgs.xkcd.com/comics/cast_iron_pans.png",
  "title": "Cast Iron Pan",
  "day": "20"
}

That's a pretty straightforward schema with nothing fancy in the field resolvers. We’d basically build the link if it’s empty:

  link: data => data.link || `https://xkcd.com/${data.num}/`,

[jlengstorf]

In progress here: gramps-graphql/gramps#46