Add shared files to collection

At the moment the collection feature can only be used for the files within the user home directory. I think it would be beneficial to have that option available for all files a user has access to. The current database setup could definitely handle that. 

In my use case, users can only access most directories via a share so they can still upload files to their own home directory individually. This way they cannot create a collection with either their own and/or shared files which can be shared with third parties via weblink.