Convert Durable Column to popup chat in footer

Summary: This feels pretty reasonable with little effort, and I think I'd use it more than the full column.

Test Plan:
Chat a lot on various pages.... still some quicksand quirks around various pages.

{F1853487}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: scp, Korvin

Differential Revision: https://secure.phabricator.com/D16627