Maniphest - fix a bug when selecting all tasks and then dragging them around

Summary: Fixes T4006.

Test Plan: clicked "select all" and dragged around tasks. Noted the task remained selected as I re-ordered, thus keeping hte count accurate. Verified when I hit "batch edit" the right tasks showed up.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4006

Differential Revision: https://secure.phabricator.com/D7566