(core) Migrate Attachments columns from marshalled blobs to JSON

Summary: Adds a migration in preparation for future work on tracking and deleting attachments. This includes a `_grist_Attachments.timeDeleted` column which isn't used yet, and changing the storage format of user columns of type `Attachments`. DocStorage now treats Attachments like RefList in general (since they use JSON), which also prompted a tiny bit of refactoring.

Test Plan: Added a migration test case showing the change in format.

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D3352

app/common/schema.ts

@@ -4,7 +4,7 @@ import { GristObjCode } from "app/plugin/GristData";
 
 // tslint:disable:object-literal-key-quotes
 
-export const SCHEMA_VERSION = 27;
+export const SCHEMA_VERSION = 28;
 
 export const schema = {
 
@@ -148,6 +148,7 @@ export const schema = {
     fileSize            : "Int",
     imageHeight         : "Int",
     imageWidth          : "Int",
+    timeDeleted         : "DateTime",
     timeUploaded        : "DateTime",
   },
 
@@ -338,6 +339,7 @@ export interface SchemaTypes {
     fileSize: number;
     imageHeight: number;
     imageWidth: number;
+    timeDeleted: number;
     timeUploaded: number;
   };