* English shouldn't fallback to any other locale
* Calculate fallback for default locale if it isn't English (useful for en_US)
* Reuse the fallback locale list when outputting translations to JavaScript
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
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
Since I18n has a DB backend now, I've introduced a helper we can use to
skip overrides in certain situations. Otherwise migration from empty
databases was broken.
