R1:9afb186c77d3

Move Ponder to AphrontSideNavFilterView

Summary: AphrontSideNavView is oldschool and I want to kill it.

Test Plan: Viewed Ponder, clicked both nav options.

Reviewers: pieter, vrana, btrahan

Reviewed By: pieter

CC: aran

Differential Revision: https://secure.phabricator.com/D3430