Modernize Feed and Phlux ordering/paging

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

Test Plan: Paged through Phlux and Feed.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7803

Differential Revision: https://secure.phabricator.com/D12359