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.

tests/app/test_navigation.py

@@ -310,6 +310,7 @@ EXCLUDED_ENDPOINTS = tuple(map(Navigation.get_endpoint_with_blueprint, {
     'user_profile_mobile_number_confirm',
     'user_profile_name',
     'user_profile_password',
+    'user_profile_security_keys',
     'using_notify',
     'verify',
     'verify_email',