LC12: Retraction of recordings from Opencast

[abrahammartin]

Opencast allows users to retract an already publish media item. This is because the user do not want to publish the video (delete) or because it has edited the video and a new version is available to be published. In those cases the old record in OAIPMH should appear as deleted (we need to confirm this) and thus we should have to delete it from the Media Platform as well.