Limit expirations for each cron execution to 50.

This should prevent servers from being crushed. 50/min is a pretty good rate.

lib/pleroma/activity_expiration.ex

@@ -46,6 +46,7 @@ defmodule Pleroma.ActivityExpiration do
 
     ActivityExpiration
     |> where([exp], exp.scheduled_at < ^naive_datetime)
+    |> limit(50)
     |> Repo.all()
   end