a private instance made public won't work out of the box

When following this workflow:

• create a private instance
• switch it to a public instance
• try to access it

You'll end up with a very ugly traceback because the anon user doesn't exist.

This shouldn't happen.

Here are proposed solutions:

• when starting a public instance, check if the anon user exist, otherwise autocreate it
• have a command to change the instance from public to private