(core) Grace period and delete-only mode when exceeding row limit

Summary:
Builds upon https://phab.getgrist.com/D3328

- Add HomeDB column `Document.gracePeriodStart`
- When the row count moves above the limit, set it to the current date. When it moves below, set it to null.
- Add DataLimitStatus type indicating if the document is approaching the limit, is in a grace period, or is in delete only mode if the grace period started at least 14 days ago. Compute it in ActiveDoc and send it to client when opening.
- Only allow certain user actions when in delete-only mode.

Follow-up tasks related to this diff:

- When DataLimitStatus in the client is non-empty, show a banner to the appropriate users.
- Only send DataLimitStatus to users with the appropriate access. There's no risk landing this now since real users will only see null until free team sites are released.
- Update DataLimitStatus immediately in the client when it changes, e.g. when user actions are applied or the product is changed. Right now it's only sent when the document loads.
- Update row limit, grace period start, and data limit status in ActiveDoc when the product changes, i.e. the user upgrades/downgrades.
- Account for data size when computing data limit status, not just row counts.

See also the tasks mentioned in https://phab.getgrist.com/D3331

Test Plan: Extended FreeTeam nbrowser test, testing the 4 statuses.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3331

app/gen-server/entity/Document.ts

@@ -53,6 +53,9 @@ export class Document extends Resource {
   @Column({name: 'removed_at', type: nativeValues.dateTimeType, nullable: true})
   public removedAt: Date|null;
 
+  @Column({name: 'grace_period_start', type: nativeValues.dateTimeType, nullable: true})
+  public gracePeriodStart: Date|null;
+
   @OneToMany(type => Alias, alias => alias.doc)
   public aliases: Alias[];
 

app/gen-server/entity/Product.ts

@@ -37,7 +37,8 @@ export const teamFreeFeatures: Features = {
   maxDocsPerOrg: 20,
   snapshotWindow: { count: 1, unit: 'month' },
   baseMaxRowsPerDocument: 5000,
-  baseMaxApiUnitsPerDocumentPerDay: 5000
+  baseMaxApiUnitsPerDocumentPerDay: 5000,
+  gracePeriodDays: 14,
 };
 
 /**