Update locale files for Djblets.

This change bundles together three things:
* Extracted all newly-introduced strings.
* Added all translations and updated strings from transifex (including a new
  Korean localization)
* Removed LANGUAGES from settings.py, so that we can just fall back on Django's
  list, which is much more comprehensive.

Reviewed at https://reviews.reviewboard.org/r/5742/