Allow users to choo choo choose a room color

Summary: This adds some basic per user / per room theming for Conpherence, which should hopefully let users identify rooms from just the sidebar color.

Test Plan: Lots of threads with different colors.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17747