Update locale files for Review Board.

This change bundles together three things:

* Extracted all newly-introduced strings.
* Added all translations and updated strings from transifex (including a few new
  languages that have small numbers of strings)
* 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/5743/