batches: implement warning banners when webhooks aren't configured
Created by: LawnGnome
Eventually closes #24310 (closed).
TODO
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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
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Consider if there's better method naming for the new store methods -
Maybe split into two PRs, because this turned into a monster?