Summary:
Ref T12733. Ref M1476. This adds `PHUIXButtonView`, for client-side button rendering.
It also adds a PHUIX example which renders the server and client versions of each component side-by-side so it's easier to see if they're messed up.
Test Plan: {F4984128}
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T12733
Differential Revision: https://secure.phabricator.com/D18051
