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could leave the review counts out of date.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 uses a pre_delete handler to fix up the review counts
when deleting one or more repositories.

Testing done:
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.

Fixes bug 3192.

Reviewed at https://reviews.reviewboard.org/r/5343/