Summary:
Attachments are a special case for granular access control. A user is now allowed to read a given attachment if they have read access to a cell containing its id. So when a user writes to a cell in an attachment column, it is important that they can only write the ids of cells to which they have access. This diff allows a user to add an attachment id in a cell if:
* The user already has access to that a attachment via some existing cell, or
* The user recently updated the attachment, or
* The attachment change is from an undo/redo of a previous action attributed to that user
Test Plan: Updated tests
Reviewers: georgegevoian, dsagal
Reviewed By: georgegevoian, dsagal
Differential Revision: https://phab.getgrist.com/D3681
ea71312d0e