SIWA: Make sure we have sent the apple user info to the server before redirecting

[Tug]

Initial attempt was reverted (before hitting production) as nginx isn't actually configured to forward /sign-in/apple/callback to calypso
Update: we don't need to deploy that change as /log-in/apple/callback is now whitelisted

Changes proposed in this Pull Request

• This PR updates how we parse the JSON user object coming from apple as there seems to have been some changes around this.
• It also refactors the sign in with apple server side code and makes sure the usersSocialNew request is completed before redirecting the user

Testing instructions

• Run this branch locally npm start
• Disable your a11n proxy and proxy https://wordpress.com to http://calypso.localhost:3000
• Try signing in with apple to an existing account from https://wordpress.com/log-in
• Optional: try signing up from https://wordpress.com/log-in and/or https://wordpress.com/start/user after removing your Apple ID association with WordPress.

[matticbot]

Test live: https://calypso.live/?branch=update/siwa-change-login-return-url

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~20 bytes added 📈 [gzipped])

name      parsed_size           gzip_size
security        +63 B  (+0.0%)      +20 B  (+0.0%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[nsakaimbo]

Paired with @Tug to test these changes, confirmed this patch is working as expected. LGTM.

[Tug]

Thanks @nsakaimbo :)