Add interface for deleted files

[JammingBen]

In order to clean up our Resource interface a bit we should add a dedicated interface for deleted resources that extends the existing Resource.

The new interface needs to contain the sdate as non-optional property, which can also be used to determine if a resource has been deleted (helper method with type predicate).

Whoever is tackling this, please also look if we can replace any trash route checks in favour of the new helper.