Add additional padding to editor

Summary: One of Tabitha's suggestions, this adds more padding to perseus editors, which I believe is contained to the question area, answer area, and hints. From {F13626} to {F13627}

Test Plan:
1) Make sure it wraps correctly still
2) Make sure that the green-overlay for widgets is correctly placed

Reviewers: alpert

Reviewed By: alpert

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D7038