[MailSystem] Closing with time args

[SharkyTheKing]

Allow for the ability to set a timer to close a channel if it's been inactive (from both staff sending/user sending).

Few ways we could do this.

1. Check for new contents from the bot that follows given formats.
2. Dispatch an event every time the command is used/given for the bot to be aware of what channel it's tied to.
3. That or ignore inactivity and just have timed close option that can only be stopped on the staff's side.