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 @rande Perhaps I'm misunderstanding the mapping here but I am still unclear as to why this shouldn't be the default.

The mapping change makes it so if you explicitly delete a media via the admin interface it works instead of throwing a database-level exception up if its in a gallery due to a foreign key constraint on the join table gallery_has_media. It shouldnt matter whether the media is in another gallery because the user is explicitly trying to delete the media. 

Is this a concern due to an interface where you can associate the media with another entity that is being admined through a sonata_model field? If so the functionality of the model field shouldnt be deleting the media, it should just clear out the relationship of the object (like a news item) to it.