Create blacklist for "view-only" datasets

[redekok]

A few datasets (de and dk) are outdated and have been replaced by newer datasets (DE_germany and DK_denmark). These datasets aren't available anymore in the front-end. However, API users can still create scenarios for these outdated datasets. Hence, it would be useful to create a blacklist for these datasets. Datasets on the blacklist should be "view-only", meaning that existing scenarios can still be viewed but no new scenarios can be created based on these datasets.