Refactor Devise integration

- Use scoped users
- Use the named route helpers instead of hardcoded paths

test/integration/admin/protected_area_test.rb

@@ -0,0 +1,22 @@
+require 'test_helper'
+
+class AdminAreaProtectedAreaTest < ActionDispatch::IntegrationTest
+  def test_unauthenticated_user_is_asked_to_authenticate_when_navigating_to_protected_area
+    visit admin_domains_url
+    assert_text 'You need to sign in before continuing'
+    assert_current_path new_admin_user_session_path
+  end
+
+  def test_authenticated_user_can_access_protected_area
+    sign_in users(:admin)
+    visit admin_domains_url
+    assert_current_path admin_domains_path
+  end
+
+  def test_authenticated_user_is_not_asked_to_authenticate_again
+    sign_in users(:admin)
+    visit new_admin_user_session_url
+    assert_text 'You are already signed in'
+    assert_current_path admin_root_path
+  end
+end