Shows and removes the assets for a kq from teacheradmin interface

Show the asset list for a kq and it is also possible to remove an asset
of that list from teacheradmin interface
Added a new exlusion kq filter to both AssetResources

Filter to get the asset list not available for a kq
Enabled the possibility to add an asset to a kq from teacheradmin interface
Fix issue in addAssetAvailability

Explicitly save first new instance of AssetAvailability.