(core) lock down modification of the _grist_Attachments table

Summary:
Rows in the _grist_Attachments table have a special lifecycle,
being created by a special method, and deleted via a special
process. All other modifications are now rejected, for simplicity.

Test Plan: added test

Reviewers: dsagal, jarek

Reviewed By: dsagal, jarek

Differential Revision: https://phab.getgrist.com/D3712

app/common/ActiveDocAPI.ts

@@ -22,6 +22,7 @@ export interface ApplyUAExtendedOptions extends ApplyUAOptions {
   oldestSource?: number;  // If set, gives the timestamp of the oldest source the undo/redo
                           // action was built from, expressed as number of milliseconds
                           // elapsed since January 1, 1970 00:00:00 UTC
+  attachment?: boolean;   // If set, allow actions on attachments.
 }
 
 export interface ApplyUAResult {