Munge text to make i18n singular forms unique.

Summary:
For ngettext strings ("1 test"/"{{num}} tests"), our translation tools
get upset if we see the same singular text in two places, but with two
different plural texts ("1 test"/"{{num}} tests") vs ("1
test"/"%(num)s tests").

There were two places this happened in khan-exercises, one because
a weird line-breaking choice made two plurals look different that were
actually the same, and the other because of a distinct varname.  We
solve the second in the normal way of replacing '1' with 'varname', so
that the singular forms now differ.

Task T1562

Test Plan:
Ran
   tools/pybabel extract -F tools/babel.ini khan-exercises
without errors.

Reviewers: james, eater, alpert

Reviewed By: alpert

CC: alpert

Maniphest Tasks: T1562

Differential Revision: http://phabricator.khanacademy.org/D4139