Create HTML version of OpenAPI doc on install.

When running `npm install`, a self-contained HTML document
with the contents of the OpenAPI specification is saved as
openapi.html in the same directory as openapi.yaml.

lib/www/server/package.json

@@ -4,7 +4,8 @@
   "description": "Dougal web backend",
   "main": "index.js",
   "scripts": {
-    "test": "echo \"Error: no test specified\" && exit 1"
+    "test": "echo \"Error: no test specified\" && exit 1",
+    "postinstall": "redoc-cli bundle ./spec/openapi.yaml -o ./spec/openapi.html"
   },
   "author": "Aaltronav s.r.o.",
   "license": "UNLICENSED",