Consider renaming "blacklist/whitelist" methods to something else

[rhymes]

Following the example of rails/rails#33677 and forem/forem#7147 I'm proposing to take into consideration the replacement of words like whitelist/blacklist from carrierwave's source code.

Possible alternatives: allow/deny, valid/invalid, permitted/blocked

According to a quick search - https://github.com/carrierwaveuploader/carrierwave/search?q=blacklist+OR+whitelist&unscoped_q=blacklist+OR+whitelist - there are a few instances of this.

A possible solution would be to add alternative methods and deprecation notices and then remove them a couple of releases down the line or something like that.

I can work on the initial PR if agreed.

Thank you!

[mshibuya]

Closed by #2442.