Allow references to be created as placeholders and exclude
them from ReferenceListNodes and the ReferenceSearchWidget.
Bonus: Fix reference numbering when items are skipped (in
list nodes and search widget).
Change-Id: I8dc5146c21f309e89ff4ddd939f7c65a0c358378
ce.InternalItem used to .hide() its .$element, but it doesn't
do this any more, so we don't need to counteract this with
.show() any more either.
Bug: T87160
Change-Id: Ic367508d821cc5c65678d4c878edc84911fd8178