disappearing level up messages

[crnvl]

Add a feature that you can set how much seconds before level up messages gets automatically deleted, this is a pretty good feature if someone wants to see the new level directly from the channel where they level up

[ConnysCode]

Suggested by: 768469568779059250

[ConnysCode]

@angelsflyinhell Added property disappearafter: number to the announce model in guilds.

Behaviour

• 0 === disabled
• Otherwise, value is in seconds.

Defaults to 0