Make jsi18n javascript file a static file

This addresses #1001

Testing:
* Without installing any language pack, should work out of the box.
* Language pack install:
    * Central server:
        * Run `./manage.py update_language_packs -l aa`
    * Distributed server:
        * Add `LANGUAGE_CODE='aa'` to `local_settings.py`
        * Make sure that visiting http://localhost:8008/math/arithmetic/addition-subtraction/basic_addition/e/addition_1/ requests /static/jsi18n/aa.js and returns a 404
        * Run `./manage.py languagepackdownload.py`
        * Visit  http://localhost:8008/math/arithmetic/addition-subtraction/basic_addition/e/addition_1/ again and make sure /static/jsi18n/aa.js returns a 200