Add blank=True for Post.user_token

This was preventing tests from passing and having a blanked-out user
token, which is OK

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

board/models.py

@@ -133,7 +133,7 @@ class Post(models.Model):
     # The user token that was used to make this post. This is used for
     # verification when a user wants to delete their post. It is not
     # particularly secret and does not need to be hashed.
-    user_token = models.CharField(max_length=30, null=True)
+    user_token = models.CharField(max_length=30, null=True, blank=True)
 
     class Meta:
         permissions = [