Pipeline: Add a side effects step after the transaction finishes

This is to run things like streaming notifications out, which will
sometimes need data that is created by the transaction, but is
streamed out asynchronously.

lib/pleroma/web/activity_pub/pipeline.ex

@@ -17,6 +17,10 @@ defmodule Pleroma.Web.ActivityPub.Pipeline do
           {:ok, Activity.t() | Object.t(), keyword()} | {:error, any()}
   def common_pipeline(object, meta) do
     case Repo.transaction(fn -> do_common_pipeline(object, meta) end) do
+      {:ok, {:ok, activity, meta}} ->
+        SideEffects.handle_after_transaction(meta)
+        {:ok, activity, meta}
+
       {:ok, value} ->
         value