Auth subsystem refactoring and tweaks.

Added proper OAuth skipping for SessionAuthenticationPlug. Integrated LegacyAuthenticationPlug into AuthenticationPlug. Adjusted tests & docs.

lib/pleroma/helpers/auth_helper.ex

@@ -0,0 +1,17 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2020 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Helpers.AuthHelper do
+  alias Pleroma.Web.Plugs.OAuthScopesPlug
+
+  @doc """
+  Skips OAuth permissions (scopes) checks, assigns nil `:token`.
+  Intended to be used with explicit authentication and only when OAuth token cannot be determined.
+  """
+  def skip_oauth(conn) do
+    conn
+    |> Plug.Conn.assign(:token, nil)
+    |> OAuthScopesPlug.skip_plug()
+  end
+end