batches: implement warning banners when webhooks aren't configured

Created by: LawnGnome

Eventually closes #24310 (closed).

TODO

• User banner
• Admin banner
• Site configuration option to disable the banner
• Store tests
• Refactor the new external service connection resolver out of batch_change.go
• Consider if there's better method naming for the new store methods
• Maybe split into two PRs, because this turned into a monster?