Add some light padding to code views.

Summary: Source code output currently juts up against the border of its container, making the first and last lines harder to read, this diff adds some padding inside.

Test Plan: {F79908} {F79907}

Reviewers: epriestley, chad, #blessed_reviewers

Reviewed By: chad

CC: chad, Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7542