677 add version number to xform and endpoint to replace a form

closes #677 and #542 
I'm sorry, I didn't understand the versioning, so that examples confusing, I've updated the comment to reflect that versions are dates. On the date note, can this be a [date field](https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.DateField.auto_now_add) with `auto_now_add`?