Feature: Ban appeal functionality

[Naamloos]

• add a config value AllowBanAppeals with commands for setting this value
• add a DM-only command $appeal [serverid]
• add the following text to the ban DM: appealing this ban is possible by running $appeal [serverid]. when appealing is indeed possible
• always check whether member is actually banned when appealing

[Naamloos]

Can add a button to ban message.