Add ability to add meta tag for fediverse author attribution

Mastodon (and potentially other fediverse clients) can attribute a
shared link to a fediverse user.

This adds the ability to do so by setting the `social.fediverse_creator`
to "@user@instance.example.com".

It has the prerequisite that the website the shared link is from is on
your allow list. To set this up in Mastodon:
1. "Preferences" on the right-hand side
2. "Public profile" on the left-hand side
3. "Versification" tab in the top bar
4. In the section "Author attribution", add all relevant domains to the
   field for "Websites allowed to credit you" (one per line)
5. "Save changes"

Note that this is not retroactive and failing to set up to the correct
domain in the allow list beforehand will not update old links in post
and might even be cached for a specific link on a specific instance.

Furthermore, the docs for this feature should include the prerequisite
and its pitfall of not setting up the allowed domain beforehand.

Also, this might also be interesting to do on an article basis. It is
not for me right now.

layouts/partials/templates/opengraph.html

@@ -78,5 +78,8 @@
         <meta property="fb:admins" content="{{ . }}">
       {{- end }}
     {{- end }}
+    {{- with .fediverse_creator }}
+        <meta name="fediverse:creator" content="{{ . }}">
+    {{- end }}
   {{- end }}
 {{- end }}