Second backup dest

I just realized it's much simpler if both our hosting servers push to both our backup servers. Just make sure users on a backup server don't have permission to write to each other's home directory (so instead of one `git` user, have one user for each IndieHoster for whom the backup server runs backups.

This branch adds a second backup destination (hopefully, I haven't managed to test it yet), and also has some bugfixes which I encountered while trying it out (will split those out to a separate PR once everything works)