validate input, take care of protocol-less URIs

[xfq]

Perhaps related: https://github.com/w3c/markup-validator/issues/27

[ylafon]

Issue is that filter_var will see the protocol-less URI as invalid, so we need to make a choice on behalf of the user. Trying both http and https can be an issue if sites are not the same (less and less the case, but...)

[tripu]

Ah, I understand, @ylafon.