Summary:
This tightens down the set of referenced columns made available to
forms for dropdowns. Previous access to columns was computed at the
level of shared tables. Now it is calculated at the level of shared
sections. That means that we can now refrain from following hidden
references, and make the referred data unavailable to forms, since
they should not need it.
Test Plan: extended test
Reviewers: jarek, dsagal
Reviewed By: jarek, dsagal
Subscribers: dsagal
Differential Revision: https://phab.getgrist.com/D4234