Add meta tag to HTML output to make it obvious 404 page was generated by fast 404 generator

Attached is my not terribly graceful patch that modifies Fast404GeneratorSubscriber::onKernelResponse to instead of writing the HTML content all in one line instead takes the response, saves it to a variable, and does a very ungraceful string replace that takes and rewrites it with and the meta tag prior to it.

Now you can discern fast 404 generated HTML from standard HTML with confidence and test against it. Woo. Hoo.

I like this idea. I've committed your patch.