Handle incoming mentions

[snarfed]

Afaik right now we don't, for either AP or webmention. We should! Example: https://hub.netzgemeinde.eu/item/5ba2d4a8-d31d-4cf3-8b35-0c47f9e967d9

First step: did we get an inbox delivery for that ^ post? What did it look like?

Related: #493

[snarfed]

Wait, sounds like we do support mentions after all? #270

[snarfed]

Confirmed, incoming mentions are working. Example: https://indieweb.social/@snarfed/111223614167779635 , delivered to https://snarfed.org/about#comment-2867614 . Full AS2 activity from Mastodon:

{
  "@context": "...",
  "id": "https://indieweb.social/users/snarfed/statuses/111223614167779635/activity",
  "type": "Create",
  "actor": "https://indieweb.social/users/snarfed",
  "published": "2023-10-12T19:07:47Z",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "cc": [
    "https://indieweb.social/users/snarfed/followers",
    "https://fed.brid.gy/snarfed.org"
  ],
  "object": {
    "id": "https://indieweb.social/users/snarfed/statuses/111223614167779635",
    "type": "Note",
    "summary": null,
    "inReplyTo": null,
    "published": "2023-10-12T19:07:47Z",
    "url": "https://indieweb.social/@snarfed/111223614167779635",
    "attributedTo": "https://indieweb.social/users/snarfed",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public"
    ],
    "cc": [
      "https://indieweb.social/users/snarfed/followers",
      "https://fed.brid.gy/snarfed.org"
    ],
    "sensitive": false,
    "atomUri": "https://indieweb.social/users/snarfed/statuses/111223614167779635",
    "inReplyToAtomUri": null,
    "conversation": "tag:[indieweb.social](http://indieweb.social/),2023-10-12:objectId=81796273:objectType=Conversation",
    "content": "<p>hi hi hi <span class=\"h-card\" translate=\"no\"><a href=\"https://fed.brid.gy/r/https://snarfed.org/\" class=\"u-url mention\">@<span>[snarfed.org](http://snarfed.org/)</span></a></span></p>",
    "tag": [{
        "type": "Mention",
        "href": "https://fed.brid.gy/snarfed.org",
        "name": "@[snarfed.org](http://snarfed.org/)@[snarfed.org](http://snarfed.org/)"
    }],
    "..."
  }
}