Drop --create-admin

[rhiaro]

It would help me greatly in the Dockerising process if the server didn't need restarting without the --create-admin flag after an admin account was created. Can it just detect this on first time load please?

[rhiaro]

Maybe a good option would be a flag which lets you pass the WebID of the user you want to be admin, and it writes the root acl with this WebID if it doesn't already exist.