tidy up the marker changes

longs/models.py

@@ -449,12 +449,13 @@ class Exam(ExamBase):
     markers = models.ManyToManyField(
         settings.AUTH_USER_MODEL,
         blank=True,
+        null=True,
         help_text="Authorised markers for the exam",
         related_name="long_exam_markers",
     )
 
     double_mark = models.BooleanField(
-        default=True, help_text="Defines if an exam is expected to be double marked"
+        default=False, help_text="Defines if an exam is expected to be double marked"
     )
 
     valid_cid_users = models.ManyToManyField(