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/core/models.py

@@ -36,6 +36,11 @@ class Version(models.Model):
 	class Meta:
 		ordering = ('-tree', )
 
+	def purge_urls(self):
+		yield '/$'
+		yield 'versions.rss'
+		# FIXME: probably a lot more?
+
 
 class Country(models.Model):
 	name = models.CharField(max_length=100, null=False, blank=False)