Support searching for deleted posts

[cellio]

https://meta.codidact.com/posts/292504

You can see your own deleted posts, and users with the Curate ability can see (any) deleted posts. However, they don't show up in search results.

I don't think we always want to show deleted posts in search results. Let's add something to search syntax that lets you include deleted posts that you can see. By default we'd exclude them (like now), but with this option, you'd get both visible and deleted posts. Most of our search syntax seems to be keyword:value, not stand-alone indicators like "include-deleted", so I'm not sure what the best syntax would be here.