fix bug where document genserver hasn't started by the time the client connects to its channel

lib/poex_web/channels/pad_channel.ex

@@ -1,5 +1,8 @@
 defmodule PoexWeb.PadChannel do
   use Phoenix.Channel
+  alias Poex.Pads.DocumentDynamicSupervisor
+  alias Poex.Repo
+  alias Poex.Pads.Document
   alias Poex.Pads.DocumentServer
 
   @moduledoc """
@@ -11,6 +14,8 @@ defmodule PoexWeb.PadChannel do
   end
 
   def join("pad:" <> document_id, _params, socket) do
+    document = Repo.get!(Document, document_id)
+    DocumentDynamicSupervisor.start_document_server(document)
     contents = DocumentServer.get_contents(document_id)
 
     {:ok, %{uuid: socket.assigns.uuid, contents: contents},