Prune ProjectExportJob records

Right now we have millions of ProjectExportJob rows in the database. Most of them aren't needed.

We should be able to remove them if:

1. They are older than a week.
2. If there is a newer entry per project.

StuckExportJobsWorker already does some cleaning of the status, but maybe we want a new Sidekiq worker to do the pruning.

We could also consider moving this data out into Redis so that it's expired automatically.

Related to !102659 (merged)