Implement webmention server

[sivy]

We want to support the #indieweb webmentions spec:

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

Server:

• [ ] Provide discovery (header, link, a)
• [ ] Receive basic webmentions
  • will need entire design for persistence 😡
• [ ] Receive "special" webmentions (ie Likes)

[sivy]

In discussions on indieweb-dev, might try implementing webmention replies in jekyll-compatible data files