Exercises in Internet Explorer show an autocomplete causing confusion to some users

Test Plan:
- Loaded various exercises and made sure inline answers and answers in the answer area have the attribute.
- Loaded in IE and verified that autocomplete no longer shows up

Reviewers: john

Reviewed By: john

Subscribers: pamela

Maniphest Tasks: T2469

Differential Revision: http://phabricator.khanacademy.org/D8839