Add page for security keys with stubbed data

This adds a new platform admin settings row, leading a page which
shows any existing keys and allows a new one to be registered. Until
the APIs for this are implemented, the user API client just returns
some stubbed data for manual testing.

This also includes a basic JavaScript module to do the main work of
registering a new authenticator, to be implemented in the next commits.

Some more minor notes:

- Setting the headings in the mapping_table is necessary to get the
horizontal rule along the top (to match the design).

- Setting caption to False in the mapping_table is necessary to stop
an extra margin appearing at the top.

app/models/user.py

@@ -10,6 +10,7 @@ from app.models.roles_and_permissions import (
     all_permissions,
     translate_permissions_from_db_to_admin_roles,
 )
+from app.models.webauthn_credential import WebAuthnCredential
 from app.notify_client import InviteTokenError
 from app.notify_client.invite_api_client import invite_api_client
 from app.notify_client.org_invite_api_client import org_invite_api_client
@@ -343,6 +344,11 @@ class User(JSONModel, UserMixin):
             '@nhs.uk', '.nhs.uk', '@nhs.net', '.nhs.net',
         ))
 
+    @property
+    def webauthn_credentials(self):
+        return [WebAuthnCredential(json) for json in
+                user_api_client.get_webauthn_credentials_for_user(self.id)]
+
     def serialize(self):
         dct = {
             "id": self.id,

app/models/webauthn_credential.py

@@ -0,0 +1,11 @@
+from app.models import JSONModel
+
+
+class WebAuthnCredential(JSONModel):
+    ALLOWED_PROPERTIES = {
+        'id',
+        'name',
+        'credential_data',
+        'created_at',
+        'updated_at'
+    }