Add user post deletion

Users can delete their posts as long as they don't clear their cookies,
and as long as server-side user sessions are persistent.

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

board/models.py

@@ -124,6 +124,10 @@ class Post(models.Model):
     # Image width and height
     image_width = models.IntegerField(null=True, blank=True)
     image_height = models.IntegerField(null=True, blank=True)
+    # 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)
 
     class Meta:
         permissions = [