I18nize the number-line widget button values.

Test Plan:
make subperseus
cd ../webapp
kake/build_prod_main.py genfiles/extracted_strings/en/javascript/perseus-package/perseus-2.js.pot.pickle
tools/devshell.py
import cPickle
with open("genfiles/extracted_strings/en/javascript/perseus-package/perseus-2.js.pot.pickle", "r") as f:
    pot = cPickle.load(f)
for m in pot:
    print m.msgid

See the 3 new strings there.

Reviewers: aria, emily

Reviewed By: emily

Differential Revision: https://phabricator.khanacademy.org/D15406