Make lightbox buttons more roundy

Summary: Ref T3612. Mobilizes the new lightbox, changes large buttons to circle icons like Conpherence.

Test Plan: Click each new button on desktop, mobile.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T3612

Differential Revision: https://secure.phabricator.com/D16961