(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/common/ActiveDocAPI.ts

@@ -153,6 +153,8 @@ export interface PermissionDataWithExtraUsers extends PermissionData {
   exampleUsers: UserAccessData[];
 }
 
+export type DataLimitStatus = null | 'approachingLimit' | 'gracePeriod' | 'deleteOnly';
+
 export interface ActiveDocAPI {
   /**
    * Closes a document, and unsubscribes from its userAction events.

app/common/DocListAPI.ts

@@ -1,4 +1,5 @@
 import {MinimalActionGroup} from 'app/common/ActionGroup';
+import {DataLimitStatus} from 'app/common/ActiveDocAPI';
 import {TableDataAction} from 'app/common/DocActions';
 import {Role} from 'app/common/roles';
 import {StringUnion} from 'app/common/StringUnion';
@@ -45,6 +46,7 @@ export interface OpenLocalDocResult {
   recoveryMode?: boolean;
   userOverride?: UserOverride;
   rowCount?: number;
+  dataLimitStatus?: DataLimitStatus;
 }
 
 export interface UserOverride {

app/common/Features.ts

@@ -45,9 +45,9 @@ export interface Features {
   baseMaxRowsPerDocument?: number;  // If set, establishes a default maximum on the
                                  // number of rows (total) in a single document.
                                  // Actual max for a document may be higher.
-                                 // TODO: not honored at time of writing.
-                                 // TODO: nuances about how rows are counted.
   baseMaxApiUnitsPerDocumentPerDay?: number;  // Similar for api calls.
+
+  gracePeriodDays?: number;  // Duration of the grace period in days, before entering delete-only mode
 }
 
 // Check whether it is possible to add members at the org level.  There's no flag