Fix scientific mantissa to not use localeToFixed

Summary:
`scientificMantissa` was being used to do math, but `localeToFixed`
was being called on its result, which prevented the math from working in
locales where commas are used instead of decimal points. This fixes that. Fixes
T1057

Test Plan:
Run `make i18n`, open the
multiplying_and_dividing_scientific_notation exercise on dev appserver with
lang=da. Ensure that the question and all hints are rendered correctly (no
`NaN`s, and all the numbers have commas instead of decimal points) and that the
correct answer is accepted.

Reviewers: csilvers

Reviewed By: csilvers

CC: alpert

Maniphest Tasks: T1057

Differential Revision: http://phabricator.khanacademy.org/D2781