This is quite a heavy thing to run, since it goes over all files of a
project, and then goes over every document in (almost) every collection
which has a property 'project' that's set to the project ID. It then goes
over every document to find all ObjectIDs and removes those from the set
of file ObjectIDs for that project. The remaining ObjectIDs are considered
orphans.
This is a very thorough search, but it doesn't require any knowledge of
the document and collection structure, so it should be future-proof.
