Support Apple Sign In

[davidje13]

Alongside the google & github sign in currently offered.

• https://developer.apple.com/sign-in-with-apple/
• https://developer.okta.com/blog/2019/06/04/what-the-heck-is-sign-in-with-apple
• https://developer.apple.com/documentation/signinwithapplerestapi

This appears to need a paid Apple Developer account, so probably won't be implemented.

[davidje13]

Migrated to subproject