Make add_ka_links work with language channels

Summary:
- `-l` command-line parameter
- Add/remove annotations in many languages
- Add `keywords` to `YouTubeVideo`
- Only send updated fields to YouTube
- Filter one-letter keywords that appear to break YouTube
- Encode video update data in UTF-8

Test Plan: Ran it on the ES and PT language channels and it added links.

Reviewers: marcia

Reviewed By: marcia

Differential Revision: http://phabricator.khanacademy.org/D8841