(core) deal with write access for attachments

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

app/client/widgets/AttachmentsEditor.ts

@@ -203,6 +203,7 @@ export class AttachmentsEditor extends NewBaseEditor {
       ...this._docComm.getUrlParams(),
       name: filename,
       ...cell,
+      maybeNew: 1,  // The attachment may be uploaded by the user but not stored in the cell yet.
       attId,
       ...(inline ? {inline: 1} : {})
     });