Extract method to generate app host in tests

On JavaScript tests, Rails URL methods don't include the port when
invoked from a test, but they do when invoked from the browser. This was
causing some tests to fail with Selenium.

spec/rails_helper.rb

@@ -34,7 +34,7 @@ end
 Capybara.register_driver :headless_chrome do |app|
   capabilities = Selenium::WebDriver::Remote::Capabilities.chrome(
     "goog:chromeOptions" => {
-      args: %W[headless no-sandbox window-size=1200,800 proxy-server=127.0.0.1:#{Capybara::Webmock.port_number}]
+      args: %W[headless no-sandbox window-size=1200,800 proxy-server=#{Capybara.app_host}:#{Capybara::Webmock.port_number}]
     }
   )