Implementing Webmentions Vouch

I was particpating remotely in IndieWebCamp Cambridge and one of the sessions I was interested in was about the new Vouch for Webmentions.

Given that I maintain the Webmention library Ronkyuu I just knew that this would be something fun to implement.

This is also forcing me to finally finish the wiring up of my webmention handling code to the site generation code so I can test some of this out :)