Check primary email address fields on CreateUser (#556)

* Check primary email address fields on CreateUser

As this check wasn't available, uid=1 (and possibly guests too, if registration is open) is able to register new users with existing email addresses. This leads to numerous 500 errors.

* Update user.go

* Lower the email first. Then check

models/user.go

@@ -600,6 +600,15 @@ func CreateUser(u *User) (err error) {
 	}
 
 	u.Email = strings.ToLower(u.Email)
+	has, err := x.
+		Where("email=?", u.Email).
+		Get(new(User))
+	if err != nil {
+		return err
+	} else if has {
+		return ErrEmailAlreadyUsed{u.Email}
+	}
+
 	isExist, err = IsEmailUsed(u.Email)
 	if err != nil {
 		return err