Allow different default locales per tenant

Note that, for everything to work consistently, we need to make sure
that the default locale is one of the available locales.

Also note that we aren't overwriting the `#save ` method set by
globalize. I didn't feel too comfortable changing a monkey-patch which
ideally shouldn't be there in the first place, I haven't found a case
where `Globalize.locale` is `nil` (since it defaults to `I18n.locale`,
which should never be `nil`), so using `I18n.default_locale` probably
doesn't affect us.

app/models/concerns/globalizable.rb

@@ -82,7 +82,7 @@ module Globalizable
         translation_class.instance_eval do
           validates method,
                     length: options[:length],
-                    if: lambda { |translation| translation.locale == I18n.default_locale }
+                    if: lambda { |translation| translation.locale == Setting.default_locale }
         end
         if options.count > 1
           translation_class.instance_eval do