simplify secrets file format

Each secret file to be deployed is now backed by one local file.
This simplifies 'setup-secrets' and the secret definitions.
Also, with the old format it was not possible to add new secrets
to secrets.nix in a simple way.

Old secrets are automatically converted to the new format when running
nix-shell.

Using the new option 'nix-bitcoin.secrets', secrets are now directly
defined by the services that use them.

pkgs/generate-secrets/update-and-generate.nix

@@ -0,0 +1,10 @@
+{ pkgs }: with pkgs;
+
+let
+  generate-secrets = callPackage ./. {};
+in
+writeScript "make-secrets" ''
+  # Update from old secrets format
+  [[ -e secrets.nix ]] && . ${./update-secrets.sh}
+  ${generate-secrets}
+''