Convert Marketplace listings to Place listings (and vice versa)

[dkoo]

With the coming Phase 2 requirement that businesses should be created as Place listings, there could be potential confusion about the use of Marketplace vs. Place listings since Phase 1 versions allow Marketplace listings to be used as business profiles. For this reason, we should create a feature in the editor that lets admins convert Marketplace listings into Place listings, and vice versa.

Since both post types share the same meta fields, it should be fairly clean to just swap the post_type property to do the conversion. One caveat is that Curated List blocks that contained the posts pre-conversion would no longer be able to find the posts by their original post type, so we'll need a strategy to handle that scenario.

cc @joeboydston