Replace toFixed with localeToFixed

Summary:
Replace all the instances of toFixed with localeToFixed in the
exercises. Also, include math-format in the exercises.

Test Plan:
Visit the exercises in local mode, ensure that the there are no
errors and that the numbers are formatted correctly.

Reviewers: john, csilvers

Reviewed By: csilvers

CC: alpert

Maniphest Tasks: T1038

Differential Revision: http://phabricator.khanacademy.org/D2520