The path processing system now works with a starting slash, also the system.site:page.front, page.403 and page.404

Path processors

The path processing system now internally deals with starting slashes, so both
InboundPathProcessorInterface and OutboundPathProcessorInterface now get a $path with a starting '/'.

Path aliases

This also results in path aliases having stored a starting 'slash'. So {url_alias}.source and {url_alias}.alias both now have a starting slash.

System.site

The system.site configuration entries now also have a starting slash, so both \Drupal::config('system.site')->get('page.403'),
\Drupal::config('system.site')->get('page.404') and \Drupal::config('system.site')->get('page.front') as well.