Add post and image deletion

This one was kind of a doozy. This also adds a custom 403 error page and
fixes some permission denied behavior that I was having issues with for
a while.

This is also set up in a way that hopefully will allow me to easily
implement user post deletion.

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

board/forms.py

@@ -155,6 +155,10 @@ class BanForm(ModelForm):
         self.instance.expires = expires
 
 
+class PostDeleteForm(forms.Form):
+    image_only = forms.CharField(widget=forms.HiddenInput(), initial="0")
+
+
 class PostModifyForm(ModelForm):
     """
     A form used to modify the attributes of a post (sticky, locked, sink, etc)