Have the linter use the utf-8 aware html parser as well, and share the code.

Test Plan:
./lint_i18n_strings_test.py
python ./clean-exercises.py ../exercises/khan-exercise.html
   -- resulted in no changes to khan-exercise.html

Reviewers: john

Reviewed By: john

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D2404