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
