ci: Generate static content for demo index at deploy time (#4119)

Rather than runtime-querying of appengine versions within the
appengine environment, we can instead generate the index at deployment
time (from git tags) and just serve static content.  This simplifies
the system and avoids dependence on Google Cloud.

This was less feasible before we adopted GitHub Actions, but is now
relatively simple.  The index will be regenerated when the index code
is updated or when a new release is created.

Closes #4074

.github/workflows/demo-version-index.yaml

@@ -9,6 +9,8 @@ on:
     paths:
       - .github/workflows/demo-version-index.yaml
       - app-engine/demo-version-index/**
+  release:
+    types: [published]
 
 jobs:
   appspot:
@@ -16,6 +18,9 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
+      - name: Generate static content
+        run: python3 app-engine/demo-version-index/generate.py
+
       - uses: google-github-actions/auth@v0
         with:
           credentials_json: '${{ secrets.APPENGINE_DEPLOY_KEY }}'