Discussions stub

A simple discussion model and controller stubs. I need it for view and for possible external service integration. For the time being it is attached, in the model, to a given FID (it could be either a district or a zone).

Later on we may add the spatial coordinates to a discussion so we may issue a spatial query to on PostGIS to see what discussion are about a given zone.
Of course we do _not_ set the default here. This is a generated file. We set the default in the migration file.