Add extension to ensure that rel="noopener" is added when target="_blank"

[hamiltonweb]

Detectify identifies a risk when a link is target="_blank".
A rel="noopener" attribute should be added.

https://support.detectify.com/customer/portal/articles/2792257-external-links-using-target-_blank-

[alanwthom]

Should we be returning "value" instead of an empty string if the parameter isn't "_blank" (i.e. "_self")? Should we look to do an insensitive case match as well (although unlikely someone would submit "_Blank" I admit)?

[owen-roberts]

Can we add some documentation & if possible some unit tests?

[owen-roberts]

I've recently had the need to split this into two separate methods for a project.
One extension method for the Link class supplied in Umbraco.Web.Models which is called using myLink.Target(), and another method which works on a string, called using myStringTarget.AsLinkTarget().

I may update the contrib library with both of these instead of LinkTarget method currently supplied.
Thoughts?