Polygon should only be reviewed if disease extent class has changed since last generation

NB. Only the final 2 commits ('Implement' and 'Tests' on Sept 15) are relevant for this story (#213). Previous commits correspond to pull request for Story #219 (https://github.com/SEEG-Oxford/ABRAID-MP/pull/189 has already been merged to master). I merged the branch in to resolve conflicts on GeoJsonDiseaseExtentFeatureProperties.