Add a "FormationView" to support dynamic flank panels

Summary:
Ref T13516. Currently, the "File Tree" element is a semi-dynamic side panel that's implemented as a special mode of a side nav panel.

This implementation is fairly clunky, and arose from organic growth out of the side nav. As such, it has some weird behaviors, doesn't have builtin support for show/hide, and can't generalize easily.

Introduce a "FormationView" which supports loading a page up with piles of side panels in various modes.

Test Plan: No callers and no user-visible impact.

Maniphest Tasks: T13516

Differential Revision: https://secure.phabricator.com/D21150