Add checkbox images for remarkup

Summary: Minor, adds some weightier checkbox styles for use in Remarkup.

Test Plan:
Test a task, Phriction, various remarkup list styles.

{F4990161}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: cspeckmim, Korvin

Differential Revision: https://secure.phabricator.com/D18080