Sync works on the deployed site; add reproducible build and deploy script

The Pages deployment is now the primary copy, so two things had to change:

- Hosted pages fall back to steam-sync.py on 127.0.0.1:8765 instead of
  refusing to sync. steam-sync.py now answers Chrome's Private Network
  Access preflight (Access-Control-Allow-Private-Network), which is what a
  public https page needs before it may call into localhost.
- The build sources lived in /tmp and would have been lost. They now live in
  build/, and build.py outputs to the repo root. deploy.sh rebuilds,
  commits and pushes in one step.

Editing index.html directly is no longer the workflow - edit build/ and run
./deploy.sh.

steam-sync.py

@@ -130,6 +130,9 @@ class Handler(http.server.SimpleHTTPRequestHandler):
         self.send_header("Access-Control-Allow-Origin", "*")
         self.send_header("Access-Control-Allow-Headers", "*")
         self.send_header("Access-Control-Allow-Methods", "GET, OPTIONS")
+        # Chrome asks for this when a public https page (e.g. the Pages deployment)
+        # calls into 127.0.0.1. Without it the request is blocked before it is sent.
+        self.send_header("Access-Control-Allow-Private-Network", "true")
 
     def do_OPTIONS(self):
         self.send_response(204)