Fix language dashboard display to trap `kalite_nphrases` key exception i...

Hi @aronasorman - this fixes #212.

Running `python manage.py update_language_packs` is supposed to fix it but it takes a long time to complete.

We now trap that the `kalite_nphrases` key exists as used on the language dashboard page template to prevent the exception.  

This is useful in case this repo is cloned and run - at least the language dashboard is shown even if the `% KA Lite Content Translated` and `% Khan Academy Content Translated` columns are empty... see attached screenshot.

I've also turned the "Language Code" column into a link if language pack for the version and language code is available so user can download the language pack.

![screenshot 2014-11-13 16 56 11](https://cloud.githubusercontent.com/assets/175580/5025874/c3091b00-6b58-11e4-92ab-5f650bb86eb0.png)