Allow "Magic Home" to be hidden

Summary: Ref T12174. Fallback behavior on this already appears to be sensible.

Test Plan:

• Hid "Magic Home".
• Viewed homepage with no dashboards on the menu.
• Saw "Magic Home" content, with no item in the menu selected, which seems reasonable.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12174

Differential Revision: https://secure.phabricator.com/D17289