Properly show the label form field's selection if it has a value upon load

Properly show the label form field's selection if it has a value upon load

This got broken during the TypeScript migration, because the constructor's
`labelId` parameter is typed as string, but in reality it was passed an
integer. This broke a `===` comparison with the `data-label-id` property of the
list items, thus failing to properly select the label.