Do not check if the asset availability is the active tab

When saving, values of asset availability were only being updated when that tab
was the only one being active, to prevent the values being overwritten when the
instance was created.

Instead of that, it will be checked that the tab is not hidden, so if the dates
are changed the new values would be stored even if the asset availability is
not the active tab.
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