(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/common/Features.ts

@@ -1,3 +1,8 @@
+export interface SnapshotWindow {
+  count: number;
+  unit: 'month' | 'year';
+}
+
 // A product is essentially a list of flags and limits that we may enforce/support.
 export interface Features {
   vanityDomain?: boolean;   // are user-selected domains allowed (unenforced) (default: true)
@@ -35,10 +40,7 @@ export interface Features {
 
   readOnlyDocs?: boolean;   // if set, docs can only be read, not written.
 
-  snapshotWindow?: {        // if set, controls how far back snapshots are kept.
-    count: number;          // TODO: not honored at time of writing.
-    unit: 'month'|'year';
-  };
+  snapshotWindow?: SnapshotWindow;  // if set, controls how far back snapshots are kept.
 
   baseMaxRowsPerDocument?: number;  // If set, establishes a default maximum on the
                                  // number of rows (total) in a single document.