The template_id field should not exposed in UpdateProcess

Found while working on frontend#1116 (closed): the API documentation for UpdateProcess and PartialUpdateProcess API endpoints shows that the template_id field is editable. It doesn't actually allow setting any value other than None though, because its queryset is set to Process.objects.none(), so you can't pick any process. All you can do is remove the association with a template that you previous applied.

It shouldn't be exposed in those APIs at all, as only the ApplyProcessTemplate and ClearProcess endpoints should be editing it.