Final cleanup / refactoring of i18n code, refactor updates to have a base.html

Have been running off of this for days, didn't realize it wasn't merged!

* Refactored i18n code, to move code that was in the central app to the i18n app
* Refactored updates, to have a `base.html` that includes relevant css and script.
* Made the `languagepackdownload` command run off the updates app.

Been running off this for a few days, so relatively confident.

@aronasorman could you make time to take a look?