LEFT JOIN instead of JOIN for ApplicationSearch ordering

Summary: Ref T4659. Because we JOIN, tasks with no value are filtered out. Instead, LEFT JOIN.

Test Plan: Issued an "Order by" and got all applicable tasks. Adjusted values and saw order change.

Reviewers: btrahan, hach-que

Reviewed By: hach-que

Subscribers: epriestley

Maniphest Tasks: T4659

Differential Revision: https://secure.phabricator.com/D10119