Show default language first and bold on video update page.
solve #1392: Return the average video size if youtube id is not found in video_file_sizes.json
Convert default_language to IETF format rather than django encoding.
Merge pull request #1421 from rtibbles/default_language

Show default language first and bold on video update page.
Merge pull request #1439 from aronasorman/1392

solve #1392: Make an estimate of the video size and indicate it's an estimate