Allow regTime in user to be blank as well

Without regTime being blank a user with regTime NULL cannot be edited
via the admin interface

contest/models.py

@@ -53,7 +53,7 @@ class User(AbstractUser):
 
     location = models.CharField(max_length=128, default="", blank=True)
     opName = models.CharField(max_length=128, default="", blank=True)
-    regTime = models.DateTimeField(null=True, default=None)
+    regTime = models.DateTimeField(null=True, default=None, blank=True)
 
     # because of cbr parsing bug, we sometimes have users who only have 70cm qsos
     # we ignore the band for them when checking QSOs