Support Webmentions

[miklb]

Background on webmentions

tl;dr

Webmention is a simple way to notify any URL when you link to it on your site. From the receiver's perspective, it's a way to request notifications when other sites link to it. Webmention is a modern update to Pingback, using only HTTP and x-www-form-urlencoded content rather than XMLRPC requests. Webmention supersedes Pingback.

There are already some open-source PHP libraries that I think could facilitate the implementation. Would also require updating core themes to support microformats2 which would be fairly trivial as it is html class names.

cc/ @Konzertheld