Refactor Mobilizon.Federation.ActivityPub and add typespecs

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/web/auth/error_handler.ex

@@ -5,6 +5,7 @@ defmodule Mobilizon.Web.Auth.ErrorHandler do
   import Plug.Conn
 
   # sobelow_skip ["XSS.SendResp"]
+  @spec auth_error(Plug.Conn.t(), any(), any()) :: Plug.Conn.t()
   def auth_error(conn, {type, _reason}, _opts) do
     body = Jason.encode!(%{message: to_string(type)})
     send_resp(conn, 401, body)