Make spatial database mandatory for SHOGun2

[buehner]

https://github.com/terrestris/shogun2/pull/198 introduced a new class that makes use of a JTS geometry, which is currently persisted as the binary representation of the Java object serialization. This is a working, but not very smart solution.

We should discuss here, if we want to make use of Hibernate Spatial in future to persist the geometries as "real" geometry data types in the database. This would require that the SHOGun2 database must be a spatial database. Afterwards, it should also be possible to visualize the geometries - for example by publishing a Layer based on the territories table from SHOGun2 in a GeoServer.