WIP method to infer if a subject is published
A possible optimisation for is_published_by_implication
test that a subject's implied publish status can be determined
add a readonly 'publish' field to subject schema
attach pubish status to outgoing json objects
only touch system_mtime for linked instances
don't force all subjects to be indexed as published
oops
move implied publication method to a mixin
trick the indexer into seeing agents with no implied publication as unpublished
oops - fix up method broken in transit
fix breaking agent test
add a field to subject factory to fix a breaking test
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
Merge pull request #648 from hudmol/subjects-and-agents-in-public

Six or Seven Degrees of Publication