Add SessionAuthenticationPlug.

lib/pleroma/plugs/session_authentication_plug.ex

@@ -0,0 +1,18 @@
+defmodule Pleroma.Plugs.SessionAuthenticationPlug do
+  import Plug.Conn
+  alias Pleroma.User
+
+  def init(options) do
+    options
+  end
+
+  def call(conn, _) do
+    with saved_user_id <- get_session(conn, :user_id),
+         %{auth_user: %{id: ^saved_user_id}} <- conn.assigns do
+      conn
+      |> assign(:user, conn.assigns.auth_user)
+    else
+      _ -> conn
+    end
+  end
+end