mrf: add anti-followbot policy

lib/pleroma/web/activity_pub/mrf/anti_followbot_policy.ex

+# Pleroma: A lightweight social networking server
+# Copyright © 2019 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.MRF.AntiFollowbotPolicy do
+  alias Pleroma.User
+
+  @behaviour Pleroma.Web.ActivityPub.MRF
+
+  # XXX: this should become User.normalize_by_ap_id() or similar, really.
+  defp normalize_by_ap_id(%{"id" => id}), do: User.get_cached_by_ap_id(id)
+  defp normalize_by_ap_id(uri) when is_binary(uri), do: User.get_cached_by_ap_id(uri)
+  defp normalize_by_ap_id(_), do: nil
+
+  defp score_nickname("followbot@" <> _), do: 1.0
+  defp score_nickname("federationbot@" <> _), do: 1.0
+  defp score_nickname("federation_bot@" <> _), do: 1.0
+  defp score_nickname(_), do: 0.0
+
+  defp score_displayname("federation bot"), do: 1.0
+  defp score_displayname("federationbot"), do: 1.0
+  defp score_displayname("fedibot"), do: 1.0
+  defp score_displayname(_), do: 0.0
+
+  defp determine_if_followbot(%User{nickname: nickname, name: displayname}) do
+    nick_score =
+      nickname
+      |> String.downcase()
+      |> score_nickname()
+
+    name_score =
+      displayname
+      |> String.downcase()
+      |> score_displayname()
+
+    nick_score + name_score
+  end
+
+  defp determine_if_followbot(_), do: 0.0
+
+  @impl true
+  def filter(%{"type" => "Follow", "actor" => actor_id} = message) do
+    %User{} = actor = normalize_by_ap_id(actor_id)
+
+    score = determine_if_followbot(actor)
+
+    # TODO: scan biography data for keywords and score it somehow.
+    if score < 0.8 do
+      {:ok, message}
+    else
+      {:reject, nil}
+    end
+  end
+
+  @impl true
+  def filter(message), do: {:ok, message}
+end