Use slug instead of preg_replace

[arxeiss]

Summary of the change:

Currently, when I set alphanumeric_filename and upload file with accents like Test - Žluťoučký kůň.pdf I got file named Test-__lu__ou________k____.pdf. In my PR I propose to use Str::slug() instead of preg_replace().

I also extended alphanumeric functionality with convert_to_alphanumeric config option. If this is set to true, then when renaming or creating folder user inputs some non-alphanum characters, everything is converted to alphanum with Str::slug() instead of poping error out.