Graphite: enable CORS for Grafana

We can't do this in Apache, because uWSGI hijacks request-processing.
See <http://stackoverflow.com/a/25783474>.

So: leverage the fact that graphite-web is a Django web app, and add a very
thin Django middleware class that sets appropriate CORS headers for origins
which match settings.CORS_ORIGINS, and add a parameter for specifying origins
to graphite::web. Nice!

Finally, tell Grafana to make GET requests to Graphite's render API and not
POST. See <https://github.com/grafana/grafana/issues/345>.

Change-Id: Ieebc69411c61bf3b08990f8890c918260da1b432
Merge "Graphite: enable CORS for Grafana" into production