Switching the MediaGoblin homepage image over to using the "official" gavroche style
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.