[DB] Use a task framework to prune leftover records
There are various tables in the aurweb database which need to
be routinely pruned; things like Sessions and Bans. There may
be more.
We can use the celery framework a task framework to schedule pruning jobs for
our web application.
This issue is being setup to track the progress or conversation on this topic. We should have a complete set of tables to prune before we work on implementing this.