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/templates/403.html

@@ -0,0 +1,8 @@
+{% extends "board/base.html" %}
+{% block title %}
+Permission denied
+{% endblock title %}
+
+{% block content %}
+<h3>You do not have permission to do this.</h3>
+{% endblock content %}