batch: implement RFC 325 for global progressive rollout

Created by: LawnGnome

High level plan:

• Add new site configuration entries to the schema. (#19796)
• Implement basic window handling functions based on that configuration: what's open, and when. (also #19796)
• Prototype scheduler. (#19797)
• Add the new "scheduled" state for changesets. (also #19797)
• Wire up the scheduler to pull from this state and enqueue. (also also #19797)
• Initial UI: "when will my changeset be processed?". (#19951)
• Documentation.