Set body classes via Quicksand config

Summary: Sends and stores additional body classes at the page level. Removes old ones, sets new ones.

Test Plan: home -> application search -> colored workboard -> config -> home with persistent chat open and minimized.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16646