FIX: Fallbacks for missing interpolation arguments

This takes effect when an interpolation is removed from a translation in
a Discourse update.

The I18n::Backend::Fallbacks loops with a catch(:exception), so calling
throw(:exception) will cause it to use the next locale, until it reaches
English which is assumed to be correct.

Also, enable fallbacks in everything except development (#3724 for more
discussion) - we should be able to test this

config/initializers/i18n.rb

@@ -28,8 +28,10 @@ class NoFallbackLocaleList < FallbackLocaleList
   end
 end
 
-if Rails.env.production?
-  I18n.fallbacks = FallbackLocaleList.new
-else
+
+if Rails.env.development?
   I18n.fallbacks = NoFallbackLocaleList.new
+else
+  I18n.fallbacks = FallbackLocaleList.new
+  I18n.config.missing_interpolation_argument_handler = proc { throw(:exception) }
 end