(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/client/models/DocPageModel.ts

@@ -14,6 +14,7 @@ import {bigBasicButton} from 'app/client/ui2018/buttons';
 import {testId} from 'app/client/ui2018/cssVars';
 import {menu, menuDivider, menuIcon, menuItem, menuText} from 'app/client/ui2018/menus';
 import {confirmModal} from 'app/client/ui2018/modals';
+import {DataLimitStatus} from 'app/common/ActiveDocAPI';
 import {AsyncFlow, CancelledError, FlowRunner} from 'app/common/AsyncFlow';
 import {delay} from 'app/common/delay';
 import {OpenDocMode, UserOverride} from 'app/common/DocListAPI';
@@ -66,6 +67,7 @@ export interface DocPageModel {
   gristDoc: Observable<GristDoc|null>;             // Instance of GristDoc once it exists.
 
   rowCount: Observable<number|undefined>;
+  dataLimitStatus: Observable<DataLimitStatus|undefined>;
 
   createLeftPane(leftPanelOpen: Observable<boolean>): DomArg;
   renameDoc(value: string): Promise<void>;
@@ -108,6 +110,7 @@ export class DocPageModelImpl extends Disposable implements DocPageModel {
   public readonly gristDoc = Observable.create<GristDoc|null>(this, null);
 
   public readonly rowCount = Observable.create<number|undefined>(this, undefined);
+  public readonly dataLimitStatus = Observable.create<DataLimitStatus|undefined>(this, null);
 
   // Combination of arguments needed to open a doc (docOrUrlId + openMod). It's obtained from the
   // URL, and when it changes, we need to re-open.
@@ -258,6 +261,7 @@ export class DocPageModelImpl extends Disposable implements DocPageModel {
       this.currentDoc.set({...doc});
     }
     this.rowCount.set(openDocResponse.rowCount);
+    this.dataLimitStatus.set(openDocResponse.dataLimitStatus);
     const gdModule = await gristDocModulePromise;
     const docComm = gdModule.DocComm.create(flow, comm, openDocResponse, doc.id, this.appModel.notifier);
     flow.checkIfCancelled();