Srb/save when logged out

Allows users to create a new tree or add to an existing tree even when they are logged out. If they click the save button while logged out, the sketch is saved to the database under the "anonymous" user. The user is then redirected to sign in or sign up. Upon sign in, the sketch is assigned out of anon's id and into the user's.

This means we can now display the canvas on the home page to new users if we want to.