Bug 1041873: Add mass deletion tool.

For a few reasons:

1) I want a summary of certain related fields for the confirmation page.
2) Cascade deletion fails when I try to mass delete certain things. A lot of relationships in the DB do not specify `ON DELETE CASCADE`, and for some reason, Django is also not attempting to cascade delete them itself.
3) This is what ops has been doing when we've asked for mass deletion, so it seemed sensible to continue with a relatively similar procedure.