JS, CSS handling in DemoBundle

# Description

This PR fixes/improves many points on JavaScript and CSS handling in the DemoBundle:

* fixes [EZP-20109](https://jira.ez.no/browse/EZP-20109)
* removes the need of node.js out of the box
* assetic now handles the filenames of the CSS and JS files to avoid some weird issues with the `cssrewrite` filter

After the merge of this PR, two rewrite rules are needed in *prod* environment, this is described in https://jira.ez.no/browse/EZP-20611

# Test
manual tests in dev and prod environment.