Redesign durable column to be more a 'mini conpherence'

Summary:
Since I plan to add collapsing, this widens the chat window and moves the switcher to the side, for more visual space for conversation.

TODO: make a magical minimizer so I can always have it open.

Test Plan:
Tested on my large display and little Macbook.

{F1854092}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16635