Have home dashboards go to single column on narrow desktops

Summary: Ref T6723. Because of the sidenav, some dashboard layouts may perform poorly on more narrow desktop displays. This provides an additional media query and CSS rules.

Test Plan:
Move viewport to 1001 px, see desktop, move to 999 pixels, see tablet-like display.

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Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6723

Differential Revision: https://secure.phabricator.com/D10956