Navigator permission changes and bug fixes

• Navigator permission changes
  » Changes the permission needed to open the navigator to buildsystem.navigator
  » Adds a permission which is needed to receive the navigator item: buildsystem.navigator.item

• Fixed bugs with the modern navigator
  » Item used to close navigator could be dropped
  » Players were able to modify other player's navigator leaving them unusable

• Minor optimisation
  » Reset unload task after world switch (or quitting) to ensure correct unload time