Major change to usernames

At the suggestion of a number of people, I've made a pretty major change today to the way that the login system works. Rather than requiring an email address, it now lets you sign up with a username instead. That way, you don't have to give us your email address unless you want to, once your account is created. It should reduce the signup barriers and improve privacy for users, both of which are good things. It also helps with some of the other stuff I will be doing this week.

If you already have a Carbon Diet account, when you next log in (using your email address) you will be prompted to choose a username to identify yourself. Once you've done that, you should use that username to log in from that time onwards.