Revert "Leave slug empty until we are sure media processing was successful."

This reverts commit f67611fb485b5a84cedc62b73beb1e551e8cb934.

For some reason, generating a slug here throws an integrity error during
a query when there is a duplicate slug.
Revert "slugs are now generated during processing"

This reverts commit 9da4e8049f2de900f7aa12f2fed8c60d6749ff0b.