Add PHUIXButtonView and a UIExample

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