Deleting an item should always delete an item #789

Core has this weird pattern where you can pass force = true when deleting a post or comment. However, force = false can produce this behavior:

• Post isn't yet trashed => Post is marked as "trash", but not deleted.
• Post is already trashed => Post is permanently deleted.

This behavior is inconsistent, and an idiosyncrasy of core internals.

In the API, DELETE <id> should always delete the item. To trash an item, clients should be expected to PUT <id> status=trash, just like any other status.

Related #618.