Delete all customize-draft posts that are referenced by a given changeset post that is deleted #329

When we save a changeset (snapshot) as a draft, we go through and transition any related auto-draft-status posts to customize-draft. This is to prevent the posts referenced in the changeset from erroneously garbage-collected when the changeset itself is persistent. However, what we're not accounting for is the case when the changeset itself is deleted without ever having been published. What we're not doing is deleting all of these related customize-draft-status posts and so they are going to be sticking around forever, orphaned from any changeset.