Allow pages to use new "flexible" nav

Summary:
This allows the nav to be laid out with divs instead of tables and for the navigation column to be made flexible. Design is non-final, this is just a step toward reactive menus that work on tablets/phones and an application menu.

I'm going to play around with flexible nav and document navigation and see if that goes anywhere.

Test Plan: Will attach screenshots.

Reviewers: btrahan, vrana, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1569

Differential Revision: https://secure.phabricator.com/D3114