Provide a direct link to OpenPGP signature of ISO image

If someone downloaded the ISO image with DAVE in Tor Browser for example and gets back to the download page, then she’s not proposed to download the signature again.

Parent Task: #9323 (closed)

Original created by @sajolida on 11220 (Redmine)

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