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

galaxy/templates/galaxy/user.jinja

@@ -5,7 +5,7 @@
 {% endblock %}
 
 {% block additional_js %}
-  <script src="{{ static('webpack/galaxy/galaxy-index.js') }}" defer></script>
+  <script src="{{ static('bundled/galaxy/galaxy-index.js') }}" defer></script>
 {% endblock %}