When requiring login signatures, order documents from oldest to newest

Summary: Depends on D18788. Ref T13024. Currently, we prompt users to sign from newest to oldest. This seems less intuitive than oldest to newest.

Test Plan: Dumped document order, saw it swap to oldest-first.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13024

Differential Revision: https://secure.phabricator.com/D18789