Incoming webmention support

[amitu]

https://www.w3.org/TR/webmention/

Webmention is a simple way to notify any URL when you mention it on your site. From the receiver's perspective, it's a way to request notifications when other sites mention it.

We can collection mentions sent by others, store them somewhere, and let doc-site etc request mentions via a processor and show them in sidebar/below the article.

[amitu]

https://rknight.me/blog/adding-webmentions-to-your-site/

[amitu]

Should be handled using wasm app.