Merge branch '240.new-payment-workflow' into 'develop'

admin/create-payment-link.sh

admin/create-webhook.sh

docs/ops/README.rst

docs/ops/stripe.rst

morph/grid/local/private-keys/stripe.webhook-secret

morph/lib/issuer.nix

nixos/modules/issuer.nix

nixos/pkgs/zkapissuer/repo.json

nixos/tests/get-passes.py

nixos/tests/private-storage.nix

nixos/tests/test_privatestorage.py