Fix review counts when deleting repositories.
Removing repositories through the admin UI was hopefully the last place that

could leave the review counts out of date. Unfortunately, there's no good way

to get signalled when doing cascaded delete operations, because it's handled

entirely by the SQL server.
This change overrides ModelAdmin.log_deletion to fix up the review counts after

deleting one or more repositories.

Removed repositories that had review requests both using the individual and

bulk delete operations. Reloaded my dashboard and saw that the review counts

were correct.