Add support for incoming remote unfollows

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -241,6 +241,24 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
     end
   end
 
+  def handle_incoming(
+        %{
+          "type" => "Undo",
+          "object" => %{"type" => "Follow", "object" => followed},
+          "actor" => follower,
+          "id" => id
+        } = data
+      ) do
+    with %User{local: true} = followed = User.get_cached_by_ap_id(followed),
+         %User{} = follower = User.get_or_fetch_by_ap_id(follower),
+         {:ok, activity} <- ActivityPub.unfollow(follower, followed, false) do
+      User.unfollow(follower, followed)
+      {:ok, activity}
+    else
+      e -> :error
+    end
+  end
+
   # TODO
   # Accept
   # Undo for non-Announce