(core) Prune snapshots outside the window in product features

Summary:
- Add a method `getSnapshotWindow` to `IInventory` and `DocSnapshotInventory`. It returns a `SnapshotWindow`, which represents a duration of time for which we keep backups for a particular document.
- `DocSnapshotPruner` calls this method and passes the window to `shouldKeepSnapshots` to determine which document versions have fallen outside the window and should be pruned.
- The implementation passed to `DocSnapshotInventory` uses a new method `getDocProduct` in `HomeDBManager` which directly returns the `Product` associated with a document, given only the document ID. Other methods in `HomeDBManager` require passing more information, especially about a user, but `DocSnapshotPruner` only knows about document IDs.

Test Plan: Added a test for `getDocProduct` and a test for `DocSnapshotPruner` where `getSnapshotWindow` is specified.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3322

app/gen-server/entity/Product.ts

@@ -1,7 +1,8 @@
 import {Features} from 'app/common/Features';
 import {nativeValues} from 'app/gen-server/lib/values';
 import * as assert from 'assert';
-import {BaseEntity, Column, Connection, Entity, PrimaryGeneratedColumn} from 'typeorm';
+import {BillingAccount} from 'app/gen-server/entity/BillingAccount';
+import {BaseEntity, Column, Connection, Entity, OneToMany, PrimaryGeneratedColumn} from 'typeorm';
 
 /**
  * A summary of features used in 'starter' plans.
@@ -149,6 +150,9 @@ export class Product extends BaseEntity {
 
   @Column({type: nativeValues.jsonEntityType})
   public features: Features;
+
+  @OneToMany(type => BillingAccount, account => account.product)
+  public accounts: BillingAccount[];
 }
 
 /**