Add delete_message_seconds to 'Guild Ban Add' event

[muddyfish]

As part of the Guild Ban Add event, it would be helpful to know how long ago messages were deleted for as part of that ban.

Giving this information would allow proxy style bots delete messages that were proxied for that user during the ban event. Right now, if a user gets banned and has messages proxied, there's no simple way to automatically delete those proxied messages, and with this proxy bots could do it themselves.