Easy language switching from video download page

This solves #227 , #1454, #1587, and #1588.

This makes the video download experience fully localized, including:
* Switching download language on the video download page directly, without needing to change the server default language
* Video titles showing above the progress bar in the localized language
* languagepackdownload command now being locale-aware

and, just for kicks:

* `videoscan` now gets called on an (async) api request, instead of within a (synchronous) view function.

![screen shot 2014-02-22 at 9 05 48 am](https://f.cloud.github.com/assets/4072455/2238232/ad22dc6c-9be3-11e3-8e58-86c10c7aca6e.png)

![image](https://f.cloud.github.com/assets/4072455/2238233/b32a2f16-9be3-11e3-95db-79919caacc1d.png)

![image](https://f.cloud.github.com/assets/4072455/2238236/cc50a286-9be3-11e3-863a-cd47bba5735c.png)