Shrink side-nav on home when durable column is open

Summary: This still needs some fine tuning, but wanted to get opinions. Using it on a laptop feels pretty good. This also moves durable-column.css into its own file since it'll likely continue to grow. Minor CSS tweaks to the near perfect rendition of durable column from pixel based mockups.

Test Plan:
Press \ on my laptop. Having issues with Chrome however, but FF and Safari work as expected.

{F322506}

Reviewers: epriestley, btrahan

Reviewed By: btrahan

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11901