Redirect old WebUI downloads page to new one

There are currently a number of sites on the Internet that link to the old
downloads_web_ui.html page, including some popular blog posts. Since the WebUI
has been moved onto the Consul page itself, these links are now broken.

This commit adds a 301 (to preserve SEO) redirect from the old page to the new
one.

/cc @armon @ryanuber 

If this looks good, we should merge it ASAP so we don't loose the existing SEO.