Remove is_linked index as it's not actually needed.
Use a separate field to indicate whether a record is "published by implication"

Just to save changing the semantics of the "published" flag, which are already
a bit hairy.  We merge these two booleans in the indexer so the public view can
work as it did before (only showing indexed items whose "published" field is
true).
Add test to ensure subjects are only visible when they are linked to a published record