Support more characters in slugs, use encoded URLs throughout the system

Browsers now consistently and transparently convert between percent-encoded URLs and Unicode IRIs when needed (meaning that even Unicode in links is converted to percent-encoding for the HTTP request and back to the human-readable form for display in the address bar).

The only truly “native” solution for us would be to use percent encoding for URLs throughout the system.

More: https://github.com/getkirby/kirby/issues/5168