Implement basic varnish purging

This allows all models inherited from PgModel to specify which
URLs to purge by either setting a field or defining a function
called purge_urls, at which point they will be purged whenever
the save signal is fired.

Also implements a form under /admin/purge/ that allows for manual
purging. This should probably be extended in the future to show
the status of the pgq slaves, but that will come later.

Includes a SQL function that posts the expires to a pgq queue. For
a local deployment, this can be replaced with a simple void function
to turn off varnish purging.

pgweb/pwn/models.py

@@ -1,14 +1,21 @@
 from django.db import models
 
+from pgweb.util.bases import PgModel
+
 from datetime import date
 
-class PwnPost(models.Model):
+class PwnPost(PgModel, models.Model):
 	date = models.DateField(null=False, blank=False, default=date.today, unique=True)
 	intro  = models.TextField(null=False, blank=False)
 	content = models.TextField(null=False, blank=False)
 
 	markdown_fields = ('intro', 'content',)
 
+	def purge_urls(self):
+		yield 'community/weeklynews/$'
+		yield 'community/weeklynews/pwn%s/' % self.linkdate()
+		yield 'weeklynews.rss'
+
 	def __unicode__(self):
 		return "PostgreSQL Weekly News %s" % self.date