Make query results page slightly more generic

Summary: Ref T2625. We currently hard-code the URI; instead, derive it from the Engine. I weakened the strength of getQueryResultsPageURI to let it build from a NamedQuery or a SavedQuery, because constructing a SavedQuery for a builtin NamedQuery is a bit of a pain.

Test Plan: Clicked links on the saved queries page, got query results.

Reviewers: btrahan, blc

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2625

Differential Revision: https://secure.phabricator.com/D6060