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/conftest.py

@@ -4479,3 +4479,11 @@ def mock_get_invited_org_user_by_id(mocker, sample_org_invite):
         'app.org_invite_api_client.get_invited_user',
         side_effect=_get,
     )
+
+
+@pytest.fixture
+def webauthn_credential():
+    return {
+        'name': 'Test credential',
+        'created_at': '2017-10-18T16:57:14.154185Z',
+    }