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.

spec/components/layout/locale_switcher_component_spec.rb

@@ -101,6 +101,7 @@ describe Layout::LocaleSwitcherComponent do
   context "when not all available locales are enabled" do
     before do
       allow(I18n).to receive(:available_locales).and_return(%i[en es fr])
+      Setting["locales.default"] = "es"
       Setting["locales.enabled"] = "es fr"
     end