Summary:
for users who don't automatically have deep rights
to the document, provide them with attachment metadata only
for rows they have access to. This is a little tricky to
do efficiently. We provide attachment metadata when an
individual table is fetched, rather than on initial document
load, so we don't block that load on a full document scan.
We provide attachment metadata to a client when we see that
we are shipping rows mentioning particular attachments,
without making any effort to keep track of the metadata they
already have.
Test Plan: updated tests
Reviewers: dsagal, jarek
Reviewed By: dsagal, jarek
Differential Revision: https://phab.getgrist.com/D3722
