feature: removing incoming links to the file on delete

[nvdk]

Context
Often files are related to other resources, which are managed through mu-cl-resources.
In this case the file is created via the file-service and linked to another resource via mu-cl-resources.
The trouble arises when the file is removed, ideally any links to the file should also be removed. Currently this requires the frontend to be aware of all possible links and removing those via mu-cl-resources and only after removing the file.
This is different how mu-cl-resources handles the delete of a resource: it also removes all links to the deleted resource. This is inline with the default expectations of ember-data.

Possible solutions

1. Updating the file service so that it also removes any references to the file in the triplestore
2. Having support for delta's in the file service and doing the delete (of the metadata) via mu-cl-resource, when the file service receives the deletion it removes the file from disk.