Check for batch existence automatically

[jechols]

This supports #336, making this a sub-sub-issue of #330.

Instead of having users flag a batch as live or purged or whatever, we should just make those checks part of various jobs.

Unlike typical pipelines, however, we're going to want to make sure users can still see what's going on. They need to know that a purge or load is necessary in the first place, since that isn't automated.

• Anything that would normally set the "NeedsStagingPurge" flag should instead fire off a job that checks regularly for a purge having occurred, then does whatever would have happened when a user manually flagged it as purged. The batch status should change to something that tells users it's pending, but that they need to be manually purged before the jobs can start doing their thing.
• When a batch is ready for loading to staging or prod, same general deal, but in these cases I think we'd actually be adding jobs into the middle of an existing pipeline. Something like this:
  • Set batch status to indicate it needs load to environment X (X being a job arg)
  • Check on load into environment X
  • Add jobs that used to happen in the UI when users flagged the issue as loaded
  • Set batch status appropriately, e.g., live, or ready for QC, or whatever
  • Continue with whatever jobs used to be post-flagging, if any
• ... other stuff?

As I write this I think maybe we want automated sshing or something first. I don't want to drop what I've started here, but if automated sshing isn't awful, it makes more sense to do that first.

[jechols]

I think the upcoming ONI Agent integration will make this unnecessary. Ideally we'd actually verify batch loads and purges, but perfection is costly and it's time to move on. Sending off a command and trusting its successful completion to mean all is well... that's definitely in the "good enough" realm.