LPS-32343 DLFolders are not removed from Index

You can check with Luke that the DLFolders are not getting removed from the Lucene Index when they are definitely deleted (removed from Recycle Bin). 

The Indexable annotation requires a return object in the method. Thus I've changed the deleteFolder methods so that they return the DLFolder.

We should add specific tests for searching DLFolders and update the DLFolderTrashHandlerTest to consider DLFolders as IndexableBaseModel.

@caorongjin