Added a sound cooldown! This should prevent frustrating ping spam.

Behavior:

• Every user has a "last ping" timestamp.
  
• When a ping is sent to a recipient with sound enabled, their timestamp is updated.
• Pings will only make sound if enough time has passed since the last ping was sent.
  
• This cooldown is user-configurable with /cping prefs cooldown <int>
  
  • Measured in seconds - default is 30