[webauthn] new commits pushed by jyasskin

The following commits were just pushed by jyasskin to https://github.com/w3c/webauthn:

* Fix #609: Formally define User Handle (#616)

- Formally define User Handle
- Rename "user id" and similar terms to "user handle" everywhere
- Change name and type of `AuthenticatorAssertionResponse` field `DOMString userId` to `ArrayBuffer userHandle`
- `PublicKeyCredentialUserEntity.id` is not renamed, but it is now referred to as the "user handle"

This does not:

- Formally define the term "user account".
- Improve the privacy considerations around returning `userHandle`, as suggested in #578.
  by Emil Lundberg
https://github.com/w3c/webauthn/commit/e74d8c4da2813559086fa32a28bdc6c576d54c1f