mrf: add initial MRF.TagPolicy engine

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

+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2019 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.MRF.TagPolicy do
+  alias Pleroma.User
+  @behaviour Pleroma.Web.ActivityPub.MRF
+
+  defp get_tags(%User{tags: tags}) when is_list(tags), do: tags
+  defp get_tags(_), do: []
+
+  defp process_tag(
+         "mrf_tag:media-force-nsfw",
+         %{"type" => "Create", "object" => %{"attachment" => child_attachment} = object} = message
+       )
+       when length(child_attachment) > 0 do
+    tags = (object["tag"] || []) ++ ["nsfw"]
+
+    object =
+      object
+      |> Map.put("tags", tags)
+      |> Map.put("sensitive", true)
+
+    message = Map.put(message, "object", object)
+
+    {:ok, message}
+  end
+
+  defp process_tag(
+         "mrf_tag:media-strip",
+         %{"type" => "Create", "object" => %{"attachment" => child_attachment} = object} = message
+       )
+       when length(child_attachment) > 0 do
+    object = Map.delete(object, "attachment")
+    message = Map.put(message, "object", object)
+
+    {:ok, message}
+  end
+
+  defp process_tag(_, message), do: {:ok, message}
+
+  @impl true
+  def filter(%{"actor" => actor} = message) do
+    User.get_cached_by_ap_id(actor)
+    |> get_tags()
+    |> Enum.reduce({:ok, message}, fn
+      tag, {:ok, message} ->
+        process_tag(tag, message)
+
+      _, error ->
+        error
+    end)
+  end
+end