Fix EZP-23200: As a developer, I want to be able to add new CSS files

JIRA: https://jira.ez.no/browse/EZP-23200

# Description

The goal of this patch is to improve the CSS files handling in PlatformUI so that it's possible add a new stylesheet without changing the code.

This is based on the patches provided in [EZP-23391](https://jira.ez.no/browse/EZP-23391) and [EZP-22510](https://jira.ez.no/browse/EZP-22510) (see https://github.com/ezsystems/ezpublish-kernel/pull/810) and it's just a matter of having a configuration file where to put the path to the stylesheets

# Tests

manual test + unit tests
*SiteAccess Node*