Fixed #20003 -- Improved and extended URLValidator

Another attempt at fixing issue 20003, as it was previously attempted in #998.

This adds support for user:password in URLValidator supported URLs.

Additionally, the test suite has been improved by adding test URLs from
http://mathiasbynens.be/demo/url-regex (with a few adjustments, like
allowing local and reserved IPs).

The previous URL validation RegEx failed this test suite on 13
occasions, so I also updated the URLValidator based on
https://gist.github.com/dperini/729294 to correctly validate all URLs in
the test suite. Among other improvements, this also adds support for
IPv4 mapped IPv6 addresses like `::ffff:192.0.2.128` and support for
unicode domain names.

Docs have also been improved.
I don't think we should go so deep into validation, we opt out from numbers but at the same time we allow the whole unicode range. Unicode numbers like `๑` would happily validate therefore it's an uphill battle.

I'd opt for a vastly simplified regex to validate FQDN: `'(?:[a-z0.9\u00a1-\uffff-]+\.?)+'`.

Sure it'll let some invalid segments go through (e.g. leading/trailing hyphens) but at least it doesn't pretend of being exhaustive. Proper validation requires a parser anyway.