New command : prunesheettethers

[TheOriginalSoni]

~prunesheettethers - Runs through db and checks if the channel still exists, if not remove sheet tether

[TheOriginalSoni]

Probably not needed as a user facing command, but just a thing that runs every 1-7 days?

Or on bot start up, but that is already too bulky already, better to just run every N days

[TheOriginalSoni]

~prunetether implemeneted in 4b45271