SecureDrop i18n

- Integrates with Flask-Babel to add i18n and l10n support
- Languages are selected by Accept-Languages header or manual override
  by query parameter
- Integrates with Transifex to collaborate on translation
- Unit tests

Todo:
- build_deb_packages.sh does not compile translation files
- Relative time format is not localized
SecureDrop i18n

- Tags strings in source and journalist interface for translation
- Adds a language selection menu in page footer
Traditional Chinese translation

- 100% translation of source and journalist interface
Italian translation

- 100% translation of source and journalist interface