[SIP-4][superset-client] add SupersetClient, refactor app

[williaster]

This PR implements SIP-4, replacing all JS ajax calls with a new SupersetClient that handles all async requests using fetch 🎉

It's a big PR due to the ajax refactoring, so here's an higher-altitude summary:

SupersetClient usage*

// appSetup.js
import SupersetClient from `@superset-ui/core`;

SupersetClient.configure(...clientConfig);
SupersetClient.init(); // CSRF auth, can also chain `.configure().init();

// anotherFile.js
import SupersetClient from `@superset-ui/core`;

SupersetClient.post(...requestConfig)
  .then(({ request, json }) => ...)
  .catch((error) => ...);

*this is currently in src/packages/core/src which will become @superset-ui/core

SupersetClient handles:

• CSRF token authentication
  • it also queues requests in the case that another request is made before the token is received
  • it checks for a token before every request, an external app that uses this can detect this by catching errors, or explicitly checking SupersetClient.isAuthorized()
• timeouts (not supported by default in fetch)
• query aborts (also not supported by default in fetch, uses AbortController polyfill)
• supports the following configuration options:
  • protocol = 'http'
  • host
  • headers
  • credentials = 'same-origin' (include for non-Superset apps)
  • mode = 'same-origin' (cors for non-Superset apps)
  • timeout
• supports gets and posts. I considered adding puts/deletes but no existing ajax calls of this type are currently made
• a given request supports the following options, which met the need of every async call in the app:
  • url or endpoint
  • headers
  • body
  • timeout
  • signal (for aborting, from const { signal } = (new AbortController()))
  • for posts
    • postPayload (key values are added to a new FormData())
    • stringify whether to call JSON.stringify on postPayload values

New dependencies

It introduces the following new packages:

• abortcontroller-polyfill
• whatwg-fetch
• fetch-mock for testing

How was this tested?

• attempted to test success + failure of every endpoint that was refactored in the browser
• updated broken / ajax-containing tests
• (TODO) add SupersetClient tests

Next steps

0. finish todos
1. merge this PR, test in staging, fix any immediate bugs
2. move packages/core to superset-ui repo
3. update imports to pull from @superset-ui/core

TODO

• implement SupersetClient
• refactor all ajax calls to use SupersetClient / fetch
• refactor / fix all ajax-containing tests
• debug SQL lab polling
• remove unused SupersetClient methods like put and destroy
• refactor API
• rebase
• add SupersetClient tests
• add API documentation to package readme
• verify sqllab error links

@mistercrunch @kristw @conglei @graceguo-supercat @hughhhh @betodealmeida @michellethomas

[williaster]

warning for this, turning this off is BAD!

[kristw]

+100

[kristw]

This one total worths its own PR.

[hughhhh]

nit: error

[williaster]

this is a test / misspelling was purposeful.

[hughhhh]

:) love thunk

[williaster]

Okay so there are some concerns that this PR is too large and so we're considering the following:

1. abandon this PR
2. create a PR in the superset-ui repo with just the SupersetClient package
3. merge then publish the package
4. create a series of smaller PRs for the ajax refactors, possibly adding integration tests to them as we go.

[williaster]

Abandoning for described approach