admin: exception from `/admin/useridentity/` endpoint

From the `/admin` interface, clicking "User Management" then "Linked account identities" gives an exception from `/admin/useridentity/`:
```
TypeError: index_view() got an unexpected keyword argument 'cls'
```
The exception comes from the `flask_admin` package.  It should be investigated if this issue still occurs with the most recent Invenio packages after issue #848 is addressed.