Revisions to WebAuthN

[jrozner]

WebAuthN is currently supported but we should make some changes to improve the experience and make it better. This is a tracking issue for all the work that needs to be done.

• Build support discoverable credentials #837
• It doesn't seem like AUTH_WEBAUTHN_AUTHENTICATOR_RESIDENT_KEY is respected, this should be fixed #826
• Rename Key Name to Credential Name
• Adding a new key from the security tab does not work #825
• Add the ability to rename a credential in the security tab
• Rewrite documentation to talk through best practices and select good defaults