Add sage-like functionality a la 4chan

When reply to a post, you can uncheck "bump" and it will not bump the
thread to the top of the board.

This is useful for the OP, too, because it allows for "permasage"
functionality which is useful for moderators.

Finally, there is also "autosink" functionality. After a per-board
threshhold of posts, a thread will stop bumping.

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

board/forms.py

@@ -29,7 +29,7 @@ class ReplyForm(PostForm):
 
     class Meta:
         model = Post
-        fields = ["name", "text", "image"]
+        fields = ["name", "text", "bump", "image"]
 
     def __init__(self, *args, op, reply, **kwargs):
         super(ReplyForm, self).__init__(*args, **kwargs)