Re-enable webmentions support

[0xdevalias]

I believe as part of https://github.com/0xdevalias/devalias.net/issues/46 we ended up disabling some of the webmentions related plugins:

• https://github.com/w3c/webmention
• https://github.com/aarongustafson/jekyll-webmention_io/releases
  • https://github.com/aarongustafson/jekyll-webmention_io/issues/130
• https://github.com/aarongustafson/jekyll-webmention_io/issues/138

It would be good to re-enable these, even if it's just manually adding the appropriate tags:

• https://webmention.io/settings
  • <link rel="webmention" href="https://webmention.io/www.devalias.net/webmention" />
  • <link rel="pingback" href="https://webmention.io/www.devalias.net/xmlrpc" />

We could also add our site to this wiki page:

• https://indieweb.org/webmention.io