Add sign in with Apple

As part of the Apple submission review process, we're being required to add a sign in with Apple option to the app.

• https://developer.apple.com/app-store/review/guidelines/#sign-in-with-apple
• https://developer.apple.com/documentation/authenticationservices/implementing_user_authentication_with_sign_in_with_apple
• https://developer.apple.com/sign-in-with-apple/

Relevant code:

Add sign in with Apple to the main webapp:

• server/web/strategies/twitter.js
• server/handlers/auth-providers/twitter.js
• modules/twitter-backend/lib/index.js