Add "status" field to Dataset to descriminate between active and obsolete datasets.

The original idea was to just mark datasets as obsolete, but the fields transmitting this information in both the CKAN as well as the CSW schemata transmit more information that we decided to include in our datasets.

Status:

• Field parsed in CSW harvester
• Field parsed in CKAN harvester
• Field parsed in UVP harvesters
• Stories written for other harvesters
• Use field to moderate search ranking