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.)

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