Add users to static language group

When setting language in extra's put them in static group too. So we can use Group Router to send them to correct dialogue for Service Rating.
@smn, @imsickofmaps this maps directly to the [`get_by_name`](https://github.com/praekelt/vumi-go/blob/develop/go/apps/jsbox/contacts.py#L642-L694) resource handler. Not too familiar with this resource, but it looks like it does a `contact_store.groups.search()` and throws an error if multiple groups match the search.