feat(application_controller): add method to check if request is on app_host_legacy?

spec/helpers/application_helper_spec.rb

@@ -1,4 +1,27 @@
 describe ApplicationHelper do
+  describe 'app_host_legacy?' do
+    let(:request) { instance_double(ActionDispatch::Request, base_url: request_base_url) }
+    let(:app_host_legacy) { 'legacy' }
+    let(:app_host) { 'host' }
+
+    before do
+      stub_const("ApplicationHelper::APP_HOST_LEGACY", app_host_legacy)
+      stub_const("ApplicationHelper::APP_HOST", app_host)
+    end
+
+    subject { app_host_legacy?(request) }
+
+    context 'request on ENV[APP_HOST_LEGACY]' do
+      let(:request_base_url) { app_host_legacy }
+      it { is_expected.to be_truthy }
+    end
+
+    context 'request on ENV[APP_HOST]' do
+      let(:request_base_url) { app_host }
+      it { is_expected.to be_falsey }
+    end
+  end
+
   describe "#flash_class" do
     it { expect(flash_class('notice')).to eq 'alert-success' }
     it { expect(flash_class('alert', sticky: true, fixed: true)).to eq 'alert-danger sticky alert-fixed' }