update model & processes to track the hazardset responsible for an administrative division's hazardcategory

[fvanderbiest]

This is for https://github.com/GFDRR/thinkhazard/issues/40

[fvanderbiest]

In the current state of our work, we won't be able to link the hazardcategory_administrativedivision_table with the originating hazardset, since:

• this would imply cross-schema foreign keys
• the hazardsets are not available to the THOR frontend (they are on the processing server)

With @pgiraud we decided to add a text field to the hazardcategory_administrativedivision_table which stems from the hazardset layer sources.

[fvanderbiest]

See #22 which updates the source field