Allow deletion of media that are associated with galleries.

Right now the way the mapping is set up there is no cascade of deletes from the Media to the GalleryHasMedia. This means that if you attempt to delete Media and there are still GalleryHasMedia entities pointing at it you get a foreign key error.

Closes #576

Cascade removal from Media to GalleryHasMedia.
@dbu, You are right, the issue is also present in any sonata's bundles. but we try to solve it the same way: never delete the data. Now maybe it will be possible to add an extra check in SonataAdminBundle to force deletion of related entities explicitly.