Exposure export now requires geonode authentication

Addresses https://bugs.launchpad.net/oq-platform/+bug/1185841.

If an unauthenticated user tries to select an area and export, he/she will be asked to log in. Once they log in, I have made it so that the user is redirected back to the app and their previous selection is restored (using url parameters). This allows for somewhat direct download links. For example:

`http://mydomain.com/oq-platform2/exposure_export.html?lat1=47.18224592701489&lng1=7.915649414062499&lat2=46.65320687122665&lng2=8.8275146484375&zoom=8`

The workflow is still pretty rough, but it was reasonably simple to create and I thought it might be a useful experiment.

This branch is a derivative of https://github.com/gem/oq-platform/pull/33, which needs to land first for a clean diff.