Conpherence - make show older messages work correctly in durable column

Summary:
Fixes T8009. This was basically moving the behavior from conpherence full behavior menu into the ConpherenceThreadManager so it could be re used in the durable column.

The durable column bit has no special styles - its just a link - but it seems to work well enough. I think it could be removed altogether if / when we add some automagical scrolling back into history stuff.

Test Plan: loaded up a conpherence in a durable column and used 'show older messages' successfully. loaded up a conpherence in regular view mid transaction and used show older and show newer successfully

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8009

Differential Revision: https://secure.phabricator.com/D12816