Always collapse the left nav on desktops

Summary:
There is basically no reason for anyone to ever use the uncollapsed mode for more than the first 2 minutes of using the tool.

Delete all code related to collapse/expand.

(I'm going to add tooltips next.)

Also move the drag bar a few pixels to the right, so it does not overlap with the scrollbar on the "local" nav if there is one.

Test Plan: Viewed in desktop/tablet/phone modes.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3413