Cleans up Intro Page post-install

I realized that, post-install, the intro page should just redirect to site_url(). I considered writing an .htaccess directive to take all requests to / and push them to /public but I didn't know if that would be a bit too invasive. I can do that if we're in agreement that that's a better idea. Still, having an index file in your root is a bit more secure than relying on .htaccess files.