Remove final traces of storage system.

This removes the final remaining traces of the AppEngine-based storage system. Infrastructure that was already broken, but is now removed includes local storage (only newer browsers) and server side storage. That infrastructure powered two features: Copying blocks between levels and creating links for sharing creations, both in Turtle.

If we want those features back, we need to build them in to our server and scripts. Since starting blocks are now parameterizable via levels & we also have a way to override level parameters from the server, this should be straightforward to implement on the Ruby side.