Improve external URL validation in many ways

Problem/Motivation

While working in some issues I realized the need to improve UrlHelper::isValid()( in the file ./core/lib/Drupal/Component/Utility/UrlHelper.php

The issues I've found:

• Punycode (International Domain Names) URLs cannot be validated
• It does not accept protocol-relative URLs
• It does not take in account the allowed protocols because it does not use UrlHelper::getAllowedProtocols()
• It uses custom regex, while we could use Symfony UrlValidator which has a similar regex

Currently the Symfony UrlValidator also has some issues:

• It depends of context, which is not available at UrlHelper::isValid()
• Its code has some bugs displaying the exception's messages

Proposed resolution

• Fix Punycode validation - We could use a library for it or just PHP intl' extension with the function idn_to_ascii()
• Accept protocol-relative URLs - currently being fixed in another issue, but fully rewriting this UrlHelper::isValid()( maybe would be a better solution: #1783278: Scheme-relative URL rejected by validation →
• Validate with the allowed protocols by using UrlHelper::getAllowedProtocols()
• Check the possibility of fixing/using Symfony UrlValidator: http://symfony.com/doc/current/reference/constraints/Url.html

Remaining tasks

User interface changes

API changes

Data model changes