Modernize Differential paging/ordering

Summary: Ref T7803. Move Differential off getPagingColumn() / getReversePaging().

Test Plan: Paged Differential results.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7803

Differential Revision: https://secure.phabricator.com/D12362