Make slugifier better

[valtlai]

• Added Unicode normalization, which handles most of the slug characters. This way we don’t need to list lots of characters.
• Moved the hyphen at the end of the character classes, so it’s clearer that it represents a literal hyphen and not a character range.
• Added ß to the character list.
• Moved the character list inside the function, because it doesn’t need to be exported. (Or does it?)
• Replaced the unnecessary function with a string.

[oscarotero]

The reason to export the characters list is to allow to configure the characters substitution in _config.js file for other casuistry not covered. But I guess with the Unicode normalization (I didn't know this trick) it's not needed.