Scheduled purge of "hasn't been crawled in N days"

### Problem Description
You can schedule partial crawls, and can run with `purge_crawl_enabled: false`. This can be helpful if you have two parts to your site, one that's small and changes frequently (frequent schedule, restrictive crawl rules) and one that's more static (infrequent schedule, broader crawl rules). But if the two do not overlap, you'd want to disable purge crawl for both, to ensure you're not deleting one set while crawling the other. But then this leaves you in a situation where you have old stale pages that need to be cleaned out.

Rather than running a full crawl with a purge phase, we could instead have the ability to schedule a purge-only crawl. Effectively just a cron to do a `delete_by_query` to match docs where `last_crawled_at` is older than ``.

### Proposed Solution
Add a way to do `bin/crawl schedule `