Show magic "Home" menu item for logged-out users on public installs

Summary:
Ref T12174. Setup is:

• Allow public access.
• Don't touch the default menu.
• Visit / while logged out.

Currently, you see "magic home" as content, but don't actually see the menu item.

Instead, show the menu item.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12174

Differential Revision: https://secure.phabricator.com/D17288