Make workboard sort-order inversions more clear by using "-1 * ..." instead of "(int)-(int)"

Summary: Depends on D20279. See D20269. Agreed that explicit -1 is probably more clear.

Test Plan: Viewed boards in each sort/group order.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20281