Allow all staff users (= admin login users) to purge things from varnish

Previously we required superuser, but that seems a bit excessive. We could
limit it based on specific groups, but for now, we'll allow all staff users.

pgweb/core/views.py

@@ -176,7 +176,7 @@ def admin_pending(request):
 
 # Purge objects from varnish, for the admin pages
 @login_required
-@user_passes_test(lambda u: u.is_superuser)
+@user_passes_test(lambda u: u.is_staff)
 def admin_purge(request):
 	if request.method == 'POST':
 		url = request.POST['url']