Add activity expiration worker

This is a worker that runs every minute and deletes expired activities.
It's based heavily on the scheduled activities worker.

config/config.exs

@@ -447,6 +447,7 @@ config :pleroma, Pleroma.Web.Federator.RetryQueue,
   max_retries: 5
 
 config :pleroma_job_queue, :queues,
+  activity_expiration: 10,
   federator_incoming: 50,
   federator_outgoing: 50,
   web_push: 50,
@@ -536,6 +537,8 @@ config :pleroma, :rate_limit,
   status_id_action: {60_000, 3},
   password_reset: {1_800_000, 5}
 
+config :pleroma, Pleroma.ActivityExpiration, enabled: true
+
 # Import environment specific config. This must remain at the bottom
 # of this file so it overrides the configuration defined above.
 import_config "#{Mix.env()}.exs"