(core) Add tip for "Add New" button

Summary:
Adds a new tip for the doc menu's Add New button. The tip is
shown only when the current user is an editor or owner, and
the site is non-empty. The presence of welcome videos or
popups will also cause the tip to not be shown; it will instead
be shown the next time the doc menu is visited.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3757

app/client/models/HomeModel.ts

@@ -75,6 +75,8 @@ export interface HomeModel {
   // user isn't allowed to create a doc.
   newDocWorkspace: Observable<Workspace|null|"unsaved">;
 
+  shouldShowAddNewTip: Observable<boolean>;
+
   createWorkspace(name: string): Promise<void>;
   renameWorkspace(id: number, name: string): Promise<void>;
   deleteWorkspace(id: number, forever: boolean): Promise<void>;
@@ -154,6 +156,9 @@ export class HomeModelImpl extends Disposable implements HomeModel, ViewSettings
   public readonly showIntro = Computed.create(this, this.workspaces, (use, wss) => (
     wss.every((ws) => ws.isSupportWorkspace || ws.docs.length === 0)));
 
+  public readonly shouldShowAddNewTip = Observable.create(this,
+    !this._app.behavioralPromptsManager.hasSeenTip('addNew'));
+
   private _userOrgPrefs = Observable.create<UserOrgPrefs|undefined>(this, this._app.currentOrg?.userOrgPrefs);
 
   constructor(private _app: AppModel, clientScope: ClientScope) {