Introduce group posts

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

test/fixtures/mastodon-status-3.json

@@ -0,0 +1,34 @@
+{
+  "@context": [
+    "https://www.w3.org/ns/activitystreams",
+    "https://zoltasila.pl/schemas/litepub-0.1.jsonld",
+    {
+      "@language": "und"
+    }
+  ],
+  "actor": "https://zoltasila.pl/users/mkljczk",
+  "attachment": [],
+  "attributedTo": "https://zoltasila.pl/users/mkljczk",
+  "cc": ["https://zoltasila.pl/users/mkljczk/followers"],
+  "content": "<p><span class=\"h-card\"><a class=\"u-url mention\" data-user=\"9xUDK5nC4I2pNtyIsq\" href=\"https://framapiaf.org/@peertube\" rel=\"ugc\">@<span>peertube</span></a></span> guess you wanted to put the <a href=\"https://joinpeertube.org/en_US/news#release-2-3-0\">en_US lang link</a></p>",
+  "context": "tag:framapiaf.org,2020-07-27:objectId=39135637:objectType=Conversation",
+  "conversation": "tag:framapiaf.org,2020-07-27:objectId=39135637:objectType=Conversation",
+  "id": "https://zoltasila.pl/objects/1c295713-8e3c-411e-9e62-57a7b9c9e514",
+  "inReplyTo": "https://framapiaf.org/users/peertube/statuses/104584600044284729",
+  "published": "2020-07-27T09:37:57.202806Z",
+  "sensitive": false,
+  "source": "@peertube@framapiaf.org guess you wanted to put the [en_US lang link](https://joinpeertube.org/en_US/news#release-2-3-0)",
+  "summary": "",
+  "tag": [
+    {
+      "href": "https://framapiaf.org/users/peertube",
+      "name": "@peertube@framapiaf.org",
+      "type": "Mention"
+    }
+  ],
+  "to": [
+    "https://framapiaf.org/users/peertube",
+    "https://www.w3.org/ns/activitystreams#Public"
+  ],
+  "type": "Note"
+}