Allow reporting of stats within a polygon

Very basic implementation to let us calculate stats within a polygon.

Right now, the request is sent as a GET parameter, with a polygon converted to a set of comma-separated coordinates:

```
https://localhost:3001/surveys/123/stats?polygon=1,1,1,2,2,2,2,1
```

This is pretty ugly, and I wonder if we shouldn't rather take a POSTed GeoJSON object and use that for the query. Thoughts @prashtx?