Go for a more generic js bundling architecture

* Don't tie the output name to webpack itself
* Don't call js bundling webpack in python code
* Make the doc more generic about js bundling

subscription/templates/subscription/stats.jinja

@@ -6,7 +6,7 @@
 
 {% block head %}
   {{ super() }}
-  <script src="{{ static('webpack/subscription/stats-index.ts') }}" defer></script>
+  <script src="{{ static('bundled/subscription/stats-index.ts') }}" defer></script>
 {% endblock %}
 
 {% block content %}