Add defaults and customizations to YTL-Linux

[AirPett]

Add the following defaults and customizations to YTL-Linux to reduce the amount of manual work needed after installing to get the system to a production-ready state:

• Use Ubuntu Server 24.04.3 as base image
• Add default config for Firefox
  • Use Oma Abitti as default homepage
  • Use ktp-jako as default download an upload directory
  • Disable New Tab, First Run and Post Update pages
  • Disable language translation
• Disable keyboard and mouse integration notification popups in VirtualBox
• Pin Firefox, Naksu and Naksu 2 to the taskbar
• Make default power settings more robust (more info in code comments)
• Uninstall gnome-keyring and apport to get rid of unnecessary popups

Please note that in order to make the pinning of Firefox to the taskbar work, one must wait in the login screen approx. 5min on the first boot after installing YTL-Linux to let the Firefox snap-package to complete installing. If the first login is made too soon, the Firefox snap-package might not yet be installed completely and for some reason the icon won't appear in the taskbar later even after reboot. This is probably due to Cinnamon (maybe?) deleting the entry from the config if the pinned apps (.desktop-files) don't exist at login. There's nothing putting the entry back into the config after that, apart from pinning it manually of course.