Start new command to generate link checker reports

For generating link checker reports only the content of the Redis
database will be used. Hence, I am going to add more information to the
recorded failure, for instance the metadata original portal. This way
the report can be generated quickly without querying for additional data
from CKAN.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>