Refactor Mobilizon.Federation.ActivityPub and add typespecs

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

lib/service/error_page.ex

@@ -3,10 +3,12 @@ defmodule Mobilizon.Service.ErrorPage do
   Render an error page
   """
 
+  @spec init :: :ok | {:error, File.posix()}
   def init do
     render_error_page()
   end
 
+  @spec render_error_page :: :ok | {:error, File.posix()}
   defp render_error_page do
     content =
       Phoenix.View.render_to_string(Mobilizon.Web.ErrorView, "500.html", conn: %Plug.Conn{})