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/lists/models.py

@@ -1,16 +1,20 @@
 from django.db import models
 
-class MailingListGroup(models.Model):
+from pgweb.util.bases import PgModel
+
+class MailingListGroup(PgModel, models.Model):
 	groupname = models.CharField(max_length=64, null=False, blank=False)
 	sortkey = models.IntegerField(null=False, default=10)
 
+	purge_urls = ('community/lists/', )
+
 	def __unicode__(self):
 		return self.groupname
 	
 	class Meta:
 		ordering = ('sortkey', )
 
-class MailingList(models.Model):
+class MailingList(PgModel, models.Model):
 	group = models.ForeignKey(MailingListGroup, null=False)
 	listname = models.CharField(max_length=64, null=False, blank=False)
 	active = models.BooleanField(null=False, default=False)
@@ -18,6 +22,8 @@ class MailingList(models.Model):
 	description = models.TextField(null=False, blank=True)
 	shortdesc = models.TextField(null=False, blank=True)
 
+	purge_urls = ('community/lists/', )
+
 	@property
 	def maybe_shortdesc(self):
 		if self.shortdesc: