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/models/setting_spec.rb

@@ -260,7 +260,7 @@ describe Setting do
   end
 
   describe ".available_locales" do
-    before { allow(I18n).to receive(:available_locales).and_return(%i[de en es pt-BR]) }
+    before { allow(I18n).to receive_messages(default_locale: :de, available_locales: %i[de en es pt-BR]) }
 
     it "uses I18n available locales by default" do
       Setting["locales.enabled"] = ""
@@ -280,6 +280,12 @@ describe Setting do
       expect(Setting.enabled_locales).to eq %i[de en pt-BR]
     end
 
+    it "adds the default locale to the list of available locales" do
+      Setting["locales.enabled"] = "en es"
+
+      expect(Setting.enabled_locales).to eq %i[de en es]
+    end
+
     it "ignores extra whitespace between locales" do
       Setting["locales.enabled"] = " de  en   pt-BR "
 
@@ -293,9 +299,9 @@ describe Setting do
     end
 
     it "ignores words that don't make sense in this context" do
-      Setting["locales.enabled"] = "yes es 1234 en SuperCool"
+      Setting["locales.enabled"] = "yes de 1234 en SuperCool"
 
-      expect(Setting.enabled_locales).to eq %i[es en]
+      expect(Setting.enabled_locales).to eq %i[de en]
     end
 
     it "uses I18n available locales when no locale is available" do
@@ -304,4 +310,44 @@ describe Setting do
       expect(Setting.enabled_locales).to eq %i[de en es pt-BR]
     end
   end
+
+  describe ".default_locale" do
+    before { allow(I18n).to receive_messages(default_locale: :en, available_locales: %i[de en es pt-BR]) }
+
+    it "uses I18n default locale by default" do
+      Setting["locales.default"] = ""
+
+      expect(Setting.default_locale).to eq :en
+    end
+
+    it "allows defining the default locale" do
+      Setting["locales.default"] = "de"
+
+      expect(Setting.default_locale).to eq :de
+    end
+
+    it "handles locales which include a dash" do
+      Setting["locales.default"] = "pt-BR"
+
+      expect(Setting.default_locale).to eq :"pt-BR"
+    end
+
+    it "ignores extra whitespace in the locale name" do
+      Setting["locales.default"] = " es "
+
+      expect(Setting.default_locale).to eq :es
+    end
+
+    it "ignores locales which aren't available" do
+      Setting["locales.default"] = "fr"
+
+      expect(Setting.default_locale).to eq :en
+    end
+
+    it "ignores an array of several locales" do
+      Setting["locales.default"] = "de es"
+
+      expect(Setting.default_locale).to eq :en
+    end
+  end
 end