Replace NavContext and render_to_response with render_pgweb

render_to_response does not work on newer django, so it needs to be
replaced. And using a speicfic context actually overcomplicates things,
it's easier to just use a wrapper function. For those cases where we
don't need NavContext, just use render() (the new shortcut function from
django), which also removes the need to use RequestContext.

pgweb/lists/views.py

@@ -1,4 +1,4 @@
-from django.shortcuts import render_to_response, get_object_or_404
+from django.shortcuts import get_object_or_404
 from django.http import HttpResponse, HttpResponseForbidden
 from django.views.decorators.csrf import csrf_exempt
 from django.conf import settings