Improved `redirect` operator to support more url types

A new parameter type can be used to control how the resulting URL is generated.

It can contain one of

• abs - Generates an absolute url, with path-prefix/siteaccess
• root - Generates a relative url without path-prefix/siteaccess
• absroot - Generates an absolute url, without path-prefix/siteaccess

Additionally improvements:

• Url starting with double slashes, ie. url without a scheme are supported.
• Improved https detection, now works if behind a proxy.

For instance to ensure that an absolute url is returned:

{redirect("/about", 302, "abs")}
{redirect("https://example.org", 302, "abs")}