Moving i18n shared code into __init__.py instead.

Right now, we've got app-essential functions defined outside of the app, breaking encapsulation and making documentation confusing.

Django app/module structure tends to be: put library / shared functions into `__init__.py`.  Here, I try to follow this structure with the `i18n` app.  I've also defined this app structure in the documentation:
https://github.com/learningequality/ka-lite/wiki/Project-structure

Testing:
* Loaded the server and clicked around, changed language.
* Downloaded a video, no problem.