Introduce PhabricatorNamedQueryQuery

Summary: Adds a first-class Query object for querying NamedQueries. xzibit would be proud.

Test Plan: Updated query edit interface to use this query, verified it works correctly.

Reviewers: btrahan, blc

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2625

Differential Revision: https://secure.phabricator.com/D6051