Refactored AHAH tab loading.

I want to use the same technique for the video page, so I figured it
was worth the effort to clean it up a bit.
Slight cleanup of the LanguagePageContext.

There's no reason for this not to be a member function.
Fixed logic error when updating the video view counter
Made the video page load tabs using ajax as well
Optimized the videos page a bit.

The page was actually reasonably fast, but I did a couple things to
speed it up:

  - Call video.prefetch_languages
  - Removed stale code for no-longer functioning javascript on the page
  - Cached the comments count
Fixed exceptions when viewing a particular revision