Prefilter inaccessible shares in DefaultShareProvider::getSharedWith()

The DefaultShareProvider now does a DB-level check to find out whether
file_source is accessible at all (deleted file) or whether it's in the
trashbin of a home storage.

One small corner case where the home storage id is in md5 form cannot
be covered properly with this approach.

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>