Notify when buckets stop seeing crashes

[jschwartzentruber]

A query like this on the CrashManager signatures view shows all buckets that haven't received a crash in the past 6 months:

{
  "op": "AND",
  "permanent": false,
  "_": {
    "op": "NOT",
    "crashentry__created__gt": "2021-11-25T18:00:00Z"
  }
}

It would be useful to add a weekly cron to generate a notification if any new buckets cross a time threshold (like settings.CLEANUP_CRASHES_AFTER_DAYS). It should link to the signatures view using the above query.