Re-Setup Download Mirroring

Before the host migration in 2018 (#117 ), we had a file download mirroring setup, to reduce the load on one specific server.

I think, it still makes sense to have this distributed, but it will need to be setup fresh.

* [ ] Reimplement file download mirroring
* [ ] SSH access for artifact mirroring sync
    * [ ] Alternatively change sync to FTPS