Resolve "More intelligent collection sorting"

Closes #57 (closed), #58 (closed) and #59 (closed)

uhepp_org/uhepp_vault/models.py

@@ -38,8 +38,8 @@ class Collection(models.Model):
         choices=VISIBILITY_LEVELS,
         default=PRIVATE_LEVEL,
     )
-    created = models.DateTimeField(auto_now_add=True)
-    activity = models.DateTimeField(auto_now_add=True)
+    created = models.DateTimeField(auto_now_add=True, null=True)
+    activity = models.DateTimeField(auto_now_add=True, null=True)
 
     class Meta:
         ordering = ['-activity', Lower('title')]