delete webapps from the database (bug 1020465)

fixes [bug 1020465](https://bugzilla.mozilla.org/show_bug.cgi?id=1020465)

WIP, do not merge/review
It's not needed because we're doing bulk deletions (by calling ```.delete()``` on the queryset), which doesn't call the model's ```.delete()``` (and thus doesn't soft delete at all, but deletes for real).