Refactor Devise integration

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

test/integration/admin/logout_test.rb

@@ -0,0 +1,15 @@
+require 'test_helper'
+
+class AdminAreaLogoutTest < ActionDispatch::IntegrationTest
+  def setup
+    sign_in users(:admin)
+  end
+
+  def test_logout
+    visit admin_root_url
+    click_on 'Log out'
+
+    assert_text 'Signed out successfully'
+    assert_current_path new_admin_user_session_path
+  end
+end