Modularize dashboard layout modes (one column, two columns, etc)

Summary:
Depends on D20405. Ref T13272. Currently, the PhabricatorDashboardLayoutConfig class uses a lot of switch() statements to define layout modes.

Although I'm not planning to add thousands of new layout modes, this (and upcoming changes) can be made substantially cleaner by using a standard modular approach.

(This doesn't fully remove PhabricatorDashboardLayoutConfig yet, but that will happen soon.)

Test Plan: Edited a dashboard, saw the same layout modes as before.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20406