(core) Limit total attachment file size per document

Summary:
- Add a new parameter `Features.baseMaxAttachmentsBytesPerDocument` and set it to 1GB for the free team product.
- Add a method to DocStorage to calculate the total size of existing and used attachments.
- Add a migration to DocStorage adding an index to make the query in the above method fast.
- Check in ActiveDoc if uploading attachment(s) would exceed the product limit on that document.

Test Plan: Added test in `limits.ts` testing enforcement of the attachment limit.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3374

app/gen-server/entity/Product.ts

@@ -39,6 +39,7 @@ export const teamFreeFeatures: Features = {
   baseMaxRowsPerDocument: 5000,
   baseMaxApiUnitsPerDocumentPerDay: 5000,
   baseMaxDataSizePerDocument: 5000 * 2 * 1024,  // 2KB per row
+  baseMaxAttachmentsBytesPerDocument: 1 * 1024 * 1024 * 1024,  // 1GB
   gracePeriodDays: 14,
 };