homepage CSS refactor

[krmax44]

• [x] no visual regressions
• [x] turn specific homepage components into reusable utilities
• [x] add wrapper_classes to slider component, allowing to add a wrapper element with customizable classes around each slider element
• [x] drop px in favor of em/rem

this requires some manual class updating in CMS to avoid visual regressions (not forgetting /en):

• setting wrapper classes on sliders
• adding backdrop classes to slider parent design container
• adding hero-illustration classes to images
• adding border and shadow utility classes to columns

I checked to only remove classes that were neither used within froide/fragdenstaat_de repos nor on CMS pages