Fix default order for Differential queries

Summary:
Fixes T3781. The UI defaults to "Created" but the query defaults to "Modified". Make the two consistent.

In particular, an issue this fixes is that previously a `/differential/?authors=duck` page would show "Order: Created" but actually order by "Modified".

Test Plan: Visited `/differential/?authors=duck` and verified the revisions were ordered by creation date.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3781

Differential Revision: https://secure.phabricator.com/D6843