Don't select disabled menu items as default

Summary: Fixes T12969. If you disable "Home" but leave it at the top, we still load it.

Test Plan: Disabled "Home". Move Dashboard into first position, see correct home layout.

Reviewers: epriestley

Reviewed By: epriestley

Spies: Korvin

Maniphest Tasks: T12969

Differential Revision: https://secure.phabricator.com/D18455