Conpherence -- finish off the "show older" functionality

Summary: Fixes T2956. Ref T2399.

Test Plan: set message limit to 2 and verified "show older" showed up, and that clicking it again and again and again showed the right stuff, ultimately not showing a "show older" UI anymore.

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2399, T2956

Differential Revision: https://secure.phabricator.com/D5721