Bundle subtitles w/ language packs

This addresses #854. 

Changes:

1. Updated the `cache_translations` command to `update_language_packs` command. So it not only caches translations, but bundles the latest subtitles with them and stores metadata on both.
2. Updated `cache_subtitles` to automatically run the `update_language_packs` command when it finishes executing. When we update subtitles, we immediately update language packs. 
3. Updated the `languagedownload` command and LanguagePack model to store subtitles in the right place and store subtitle_counts.
4. Improved our utilities for looking up language names. 

Testing: 
- To test, try running the `cache_subtitles` command with a specific language. This will download subtitles for a specific language, and then run the `update_language_packs` command to download the latest translations. (you may need to run `generate_subtitle_map` before `cache_subtitles` will work.