(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/common/Features.ts

@@ -47,6 +47,8 @@ export interface Features {
                                  // Actual max for a document may be higher.
   baseMaxApiUnitsPerDocumentPerDay?: number;  // Similar for api calls.
   baseMaxDataSizePerDocument?: number;  // Similar maximum for number of bytes of 'normal' data in a document
+  baseMaxAttachmentsBytesPerDocument?: number;  // Similar maximum for total number of bytes used
+                                                // for attached files in a document
 
   gracePeriodDays?: number;  // Duration of the grace period in days, before entering delete-only mode
 }