Reintroduce pending status and also add draft status for revisions

[coreymckrill]

This allows updates to be saved without having to specify a future publish date. Add both draft and pending so that the workflow can be similar to the standard post publishing workflow in Core.

[StevenDufresne]

Did you need anything for this, or are we just holding this off for now?

[coreymckrill]

@StevenDufresne It's not a high priority right now, so I've just been squeezing it in when I have time. I do think there's quite a bit to think through for the front end. A couple things off the top of my head:

• Switching between future, pending, and draft statuses in the revision editor
• Adding pending and draft options to the custom sidebar when you click Save For Later