Add buttons to check all/none available languages

Although most Consul Democracy installations will only have a few
available languages using `config.i18n.available_locales`, there's a
chance some installation will keep every language as available and will
enable the desired ones using the admin interface. In these cases,
enabling (or disabling) every language would be tedious, particularly
when casually experimenting in a staging environment or while using the
official Consul Democracy demo.

So we're adding buttons to simplify the process. Since some
installations might have only a couple of available languages, and in
this case these buttons would be pretty much useless, we're only showing
them when there are many languages available.

spec/components/admin/locales/form_component_spec.rb

@@ -5,10 +5,9 @@ describe Admin::Locales::FormComponent do
   let(:enabled_locales) { %i[en nl] }
   let(:locales_settings) { Setting::LocalesSettings.new(default: default_locale, enabled: enabled_locales) }
   let(:component) { Admin::Locales::FormComponent.new(locales_settings) }
+  before { allow(I18n).to receive(:available_locales).and_return(%i[de en es nl]) }
 
   describe "default language selector" do
-    before { allow(I18n).to receive(:available_locales).and_return(%i[de en es nl]) }
-
     it "renders radio buttons when there are only a few locales" do
       render_inline component
 
@@ -34,4 +33,24 @@ describe Admin::Locales::FormComponent do
                                   selected: "Nederlands"
     end
   end
+
+  describe "buttons to check all/none" do
+    it "is not rendered when there are only a few locales" do
+      render_inline component
+
+      expect(page).not_to have_button "Select all"
+      expect(page).not_to have_button "Select none"
+    end
+
+    it "is rendered when there are many locales" do
+      allow(component).to receive(:select_field_threshold).and_return(3)
+
+      render_inline component
+
+      page.find(:fieldset, "Enabled languages") do |fieldset|
+        expect(fieldset).to have_button "Select all"
+        expect(fieldset).to have_button "Select none"
+      end
+    end
+  end
 end