Prepend http:// when redirecting profile links, if not present

[SergeyBiryukov]

#4753 obfuscated profile links by passing them through an internal redirection system, e.g. https://profiles.wordpress.org/website-redirect/dd32.

Some URLs lack the http:// prefix and result in a "User not found" message instead of the redirect.

To reproduce:

1. Go to https://profiles.wordpress.org/drosendo/.
2. Click the Website URL.
3. You're redirected to https://profiles.wordpress.org/website-redirect/www.rosendo.pt.
4. The page displays a "User not found" error message, instead of proceeding with the redirect.