(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/ui/HomeLeftPane.ts

@@ -38,7 +38,8 @@ export function createHomeLeftPane(leftPanelOpen: Observable<boolean>, home: Hom
         // "Add New" menu should have the same width as the "Add New" button that opens it.
         stretchToSelector: `.${cssAddNewButton.className}`
       }),
-      testId('dm-add-new')
+      dom.cls('behavioral-prompt-add-new'),
+      testId('dm-add-new'),
     ),
     cssScrollPane(
       cssPageEntry(