revisit "deleted_at" strategy

[ashleygwilliams]

indexing on deleted at adds little value for a lot of cost. let's move deleted items to their own table and remove the index. i think this will greatly improve performance while still providing the fallback.

[cadecairos]

👍

I think we also should look at making sure things properly cascade (I don't think they do right now!). i.e. a project delete also moves its child pages and elements to their deleted tables.