Set up the process and tools to allocate unique *.dl.amnesia.boum.org hostnames

It would be nice to be able to actually allocate such unique hostnames, once after #11055 (closed) is done mirror operators start asking us :)

Original created by @intrigeri on 11335 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information