(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

sandbox/grist/schema.py

@@ -15,7 +15,7 @@ import six
 
 import actions
 
-SCHEMA_VERSION = 27
+SCHEMA_VERSION = 28
 
 def make_column(col_id, col_type, formula='', isFormula=False):
   return {
@@ -235,6 +235,7 @@ def schema_create_actions():
       make_column("fileSize",     "Int"),  # The size in bytes
       make_column("imageHeight",  "Int"),  # height in pixels
       make_column("imageWidth",   "Int"),  # width in pixels
+      make_column("timeDeleted",  "DateTime"),
       make_column("timeUploaded", "DateTime")
     ]),