Fix bugs with WebAuthn preventing sign in and registration. (#22651)

Fix bugs with WebAuthn preventing sign in and registration. (#22651)

This PR fixes two bugs with Webauthn support:

* There was a longstanding bug within webauthn due to the backend using
URLEncodedBase64 but the javascript using decoding using plain base64.
This causes intermittent issues with users reporting decoding errors.
* Following the recent upgrade to webauthn there was a change in the way
the library expects RPOrigins to be configured. This leads to the
Relying Party Origin not being configured and prevents registration.

Fix #22507

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>