R1:0381eed6e619

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…