Select dubbed videos via topic page clicking.

Previously, we implemented detection of all SRT and dubbed videos and calculation of each's URL, and made those URLs available in the topic tree.  Then, in the topic and video pages, we ignored the dubbed video URLs, and hard-coded the video to show as the english URLs.

This PR allows selecting of another language's dubbed video.  It also exposes links on the topic page to select the language.  Once inside the page, the video is selected by the `lang` querystring parameter, or by the current session language.  To switch languages, a user would have to go back to the topic page.

Topic page (red background is dubbed video, and is clickable)
![image](https://f.cloud.github.com/assets/4072455/1628188/522c4a76-5707-11e3-9372-02b1c0037886.png)

Video page (note the URL)
![image](https://f.cloud.github.com/assets/4072455/1628195/6f2eae98-5707-11e3-813e-f3298227a014.png)