Added treatment of nicknames. For new users, it is now mandatory to create nickname when registering. For existing users, nickname can be set at most once. When logging in, both nicknames and email addresses are now accepted as usernames. In the user info box and on the personal pages, nicknames are now preferred to be shown instead of email addresses. As I went along I refactored some of the WebSession code, but much more refactoring would be desirable to improve this module.