Introduce PhabricatorSavedQueryQuery

Summary: Same as D6051, but for SavedQueries instead of NamedQueries. These are POLICY_PUBLIC because you need to know the hash to access them, and because we want to let users copy/paste query URLs. Ref T2625.

Test Plan: Saved a query, reused a saved query.

Reviewers: btrahan, blc

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2625

Differential Revision: https://secure.phabricator.com/D6054