Entirely replace the old filetree UI with the "flank" UI

Summary:
Ref T13516. Deletes all old filetree / flex / active / collapse nav code in favor of the new code.

Restores the inline tips in the path tree.

Test Plan: {F7374175}

Maniphest Tasks: T13516

Differential Revision: https://secure.phabricator.com/D21154