Fix two left-nav bugs

Summary:

• Fixes a bug where dragging a flexible left nav and then pressing "f" to hide it didn't properly reset the content panel's left margin.
• Fixes a bug where D4185 removed "white-space: nowrap;" accidentally (from @asherkin)

Test Plan:

• Dragged bar, then pressed "f".
• Can't repro the nowrap thing for some reason.

iiam

Reviewers: asherkin, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4194