Fix: auth process no longer matches Observable and Github

[mootari]

• Observable has split up its login flow: the first step now only requests the login name, after which a redirect to the user's external auth service (e.g. github) is issued.
• Github has ramped up its security measures: since July 2019 each new device must be verified if 2FA is disabled (announcement; user feedback: 1, 2).
• This PR introduces a dependency to cheerio, which is used to retrieve the actual form data from each page.
• It also adds a prompt for the device token, but verification currently fails with an HTTP 422 error. 2FA might just work (couldn't test myself).

Fixes #2

[Fil]

with backup.js:

const {RestClient, AuthGithubPrompt} = require('observable-client');

const client = new RestClient;
(async () => {
  await new AuthGithubPrompt(client, {
    clientId: '1a8619df27715d9d2c97',
    loginName: 'fil',
  }).authorize();

  console.log('authorized', (await client.get('/user')).body);
})();

❯ /usr/local/Cellar/node/14.8.0/bin/node backup.js
(node:45907) [DEP0066] DeprecationWarning: OutgoingMessage.prototype._headers is deprecated
(Use `node --trace-deprecation ...` to show where the warning was created)
✔ GitHub password … *********
✔ GitHub 2FA Token … 000000
(node:45907) UnhandledPromiseRejectionWarning: Error: Unprocessable Entity
    at Request.callback (/Users/fil/Source/observable/backup/node_modules/superagent/lib/node/index.js:804:15)
    at IncomingMessage.<anonymous> (/Users/fil/Source/observable/backup/node_modules/superagent/lib/node/index.js:1036:18)

[mootari]

The auth process is probably the same for 2FA tokens and device codes, and fixing one will likely fix the other.

[mootari]

I just force-pushed a fix (authenticity token was taken from the wrong form). It works for device verification, and the problem with 2FA was very likely the same. Can you please give it another try?

[Fil]

Yes! Merci 🙏