[WIP] Download Icebox input file feature

Inputs are available under `/icebox/download/input/<calculation_type/<id>`

On the filesystem files are in `MEDIA_ROOT/icebox/<user>/<id>_<calculation_type>.zip`

![screenshot from 2014-12-22 12 08 41](https://cloud.githubusercontent.com/assets/1818657/5524290/56adef46-89d3-11e4-98de-b48050300f51.png)

The `zip` file is deleted when a calculation is removed.
This PR also changes the ID showed in the icebox calculation list: before the `engine_id` was used, now we use the platform internal `id` for more consistency.

I would do a

```
if os.path.exists(filename):
    ... delete ...