Socioeconomic db

Implementation of the new socioeconomic database and of the corresponding API to retrieve data from the QGIS SVIR plugin.
Leveraging the Django admin application, it is possible (for an administrator) to visualize and update the database directly from the web (which was not possible in the previous version of the DB because it was not mapped as a Django app).

See the companion PRs:
* https://github.com/gem/qt-experiments/pull/58
* https://github.com/gem/oq-platform-data/pull/2
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