auth: introduce AuthUser.is_default_user attribute This makes makes a number of checks more readable. The username of the default user is presently hardcoded to "default" (in db.User.DEFAULT_USER); this is currently what defines the default user, and this commit doesn't change that. (Even if the check that defines is_default_user is a comparison between user IDs and not usernames, the anonymous_user object used in the comparison is loaded by looking up the user named "default".)