notify if offline & allow sketches to keep running

[therewasaguy]

• use window.navigator.onLine to display "offline" message (especially when trying to save or run a sketch as both currently require internet access). Somehow indicate that work is being saved locally and notify when internet access has returned.
• load the preview iframe as a blob so that we don't need to load it from the server and can continue to preview sketches while offline. I think this is what thimble does but need to look into it further / would love to hear ideas for how to handle this.