Return 410 for successfullyinstalled after install files are gone

It was possible to append '/successfullyinstalled' to any SilverStripe
URL (with the cms module present), and the code would run, including
republishing the 404 page.

Now, if the install.php file is gone, this controller method throws the
410 (Gone) status code. The 410 status is chosen rather than 404 to
mitigate the likelihood that crawlers such as the Googlebot will follow
stray links to this URI.
Merge pull request #700 from oddnoc/successfullyinstalled-fix

Return 410 for successfullyinstalled after install files are gone