Don't offer personal saved queries in global "Search Scope" settings dropdown

Summary: Fixes T13405. We currently offer non-global custom saved queries here, but this doesn't make sense as a global default setting.

Test Plan: Saved a global search query, edited global search settings, no longer saw the non-global query as an option.

Maniphest Tasks: T13405

Differential Revision: https://secure.phabricator.com/D20793