db/Pad.js: make "force" parameter non optional in Pad.prototype.copy()
db/Pad.js: make "force" parameter non optional in Pad.prototype.copy()

This function was simulating two overloads:
  1. copy(destinationID, force, callback)
  2. copy(destinationID, callback), in this case "force" would be assumed false

But all the call sites always used the version with arity 3.
Thus, we can remove that optionality and always assume that the funcion will be
called with three parameters. This will simplify future work.

joequant/etherpad-lite,storytouch/etherpad-lite,ether/etherpad-lite,joequant/etherpad-lite,storytouch/etherpad-lite,aptivate/etherpad-lite,ether/etherpad-lite,ether/etherpad-lite,storytouch/etherpad-lite,aptivate/etherpad-lite,aptivate/etherpad-lite,joequant/etherpad-lite,joequant/etherpad-lite,aptivate/etherpad-lite