Show default language first and bold on video update page.

Fix for issue #1389.

Changes:
* Check for default language either for system or current user (assuming that teachers with a different default language to the system setting will administer their own video downloads primarily).
* Pop default language from available languages orderedDict.
* Pass to template separately and render first in list, and in bold with {% trans "default" %} flagged.

I assume it was just on the video update page you wanted this, @bcipolli?

@aronasorman Can you review this?