Refactor User.webauthn_credentials into a ModelList

This saves a bit of repetition, and lets us attach other methods to the
collection, rather than having multiple methods on the user object
prefixed with the same name, or random functions floating about.

app/models/webauthn_credential.py

@@ -6,7 +6,8 @@ from fido2.cose import UnsupportedKey
 from fido2.ctap2 import AttestationObject, AttestedCredentialData
 from flask import current_app
 
-from app.models import JSONModel
+from app.models import JSONModel, ModelList
+from app.notify_client.user_api_client import user_api_client
 
 
 class RegistrationError(Exception):
@@ -60,3 +61,16 @@ class WebAuthnCredential(JSONModel):
             'credential_data': self.credential_data,
             'registration_response': self.registration_response,
         }
+
+
+class WebAuthnCredentials(ModelList):
+
+    model = WebAuthnCredential
+    client_method = user_api_client.get_webauthn_credentials_for_user
+
+    @property
+    def as_cbor(self):
+        return [credential.to_credential_data() for credential in self]
+
+    def by_id(self, key_id):
+        return next((key for key in self if key.id == key_id), None)