Add a global setting for controlling the default main menu search scope

Summary: Fixes T13405. The default behavior of the global search bar isn't currently configurable, but can be made configurable fairly easily.

Test Plan: Changed setting as an administrator, saw setting reflected as a user with no previous preference. As a user with an existing preference, saw preference retained.

Maniphest Tasks: T13405

Differential Revision: https://secure.phabricator.com/D20787