Added @DiscordMention parameter to !whitelist add command for staff members.

• Staff members can now add @DiscordMention to the end of the existing whitelist add command like so:
  • !whitelist add minecraftUsername @DiscordMention
• This allows staff members to keep control of the whitelisting process without requiring user input.
• With whitelisted-role-auto-add & whitelisted-role-auto-remove enabled, the bot will now add the user to the user list & assign roles automatically which previously the user would have to whitelist themselves to do.