Remove checkout records when referenced Media Object is deleted

[masaball]

Currently there is no direct link between Checkout records in the database and the media object they are referencing. This results in checkouts potentially being orphaned when a media object is deleted. For the most part this should be harmless now that we have added handling to the checkouts display to skip these entries, but it would be good to have some mechanism for clearing these out.

Options would be:

1. Implement a callback to delete checkout records when the parent media object is deleted.
2. Implement a rake task to be run manually as needed
3. Implement a rake task to be run via cron job (once a week?)

We do not have any public instances that use CDL but other institutions might, so if we choose option 1 we may still want to write a rake task to clear out any old entries that occurred before we implemented a fix.