FEATURE: Localization fallbacks (server-side)

The FallbackLocaleList object tells I18n::Backend::Fallbacks what order the
languages should be attempted in. Because of the translate_accelerator patch,
the SiteSetting.default_locale is *not* guaranteed to be fully loaded after the
server starts, so a call to ensure_loaded! is added after the locale is set for
the current user.

The declarations of config.i18n.fallbacks = true in the environment files were
actually garbage, because the I18n.default_locale was
SiteSetting.default_locale, so there was nothing to fall back to. *derp*

app/controllers/application_controller.rb

@@ -155,6 +155,8 @@ class ApplicationController < ActionController::Base
                   else
                     SiteSetting.default_locale
                   end
+
+    I18n.fallbacks.ensure_loaded!
   end
 
   def store_preloaded(key, json)