Opening existing code

[eee-c]

If there is already localStorage, then opening a shared link will overwrite the current document. A new document should be created instead.

Not sure if this is needed for the book or not.

[eee-c]

Yah, this overwrite the "3d spinning thingy", which is just awkward when subsequently using the Open dialog. Need to capture gzip content in an "Untitled" project or prompt the user to name it.