Added new config option "check_existing_bans", which checks if the user has already been banned. If they have, WhitelistCheck will not re-ban them, and will instead pass through their existing ban message.

Defaults to "true" on config.

This feature was added to help preserve ban messages of those who have been banned and removed from the whitelist, in case the ban message was notable or specific.

The "check_existing_bans" feature will not run if the user is banned and still on the whitelist.