Add lakes and rivers as predefined geometries for subscriptions

[nuest]

Data for lakes, rivers > https://github.com/kokoalberti/postgis-baselayers/tree/master/app/datasets/naturalearth/

We can use the actual river geometries because the subscriptions will be based on simple geometric overlays.

The geometries should be loaded into the database (table for predefined geometries) based on a local or remotely accessed file with a Django management command, maybe like:

python manage.py load_geometries_water

Based on #37