Cruft removal, Bug fixes, and Audio.startDrumSound() in JS

* fix memory leak in client particles, we weren't deleting the ParticleTree in the client on shutdown
* removing false color support from VoxelTreeElement to save on memory usage in server - closes #1280 
* removed lots of old debug menus that can break local rendering
* removed debug rendering of coverage maps in client
* removed various pieces of unused or barely used cruft - closes #2160 
* removed VoxelSystem::getVoxelEnclosing() - closes #2163 
* moved hydra finger painting to JS - closes #2162 
* added support for Audio.startDrumSound() to JS
* moved voxel drumming to JS - closes #2161