FIX: client's translation overrides were not working when the current locale was missing a key
FIX: ExtraLocalesController.show was not properly handling multiple translations
FIX: JsLocaleHelper#output_locale was not properly handling multiple translations
FIX: ExtraLocalesController.show's spec which was randomly failing
FIX: JsLocaleHelper#output_locale was muting cached translations hashes
REFACTOR: move 'enableVerboseLocalization' to the 'localization' initializer
REFACTOR: remove unused I18n.js methods (getFallbacks, localize, parseDate, toTime, strftime, toCurrency, toPercentage)
REFACTOR: remove all I18n.pluralizationRules and instead use MessageFormat's pluralization rules
TEST: add tests for localization initializer
TEST: add tests for I18n.js
