Batch Changes webhook detection is outdated

[eseliger]

Webhooks used to be configured in the code host connection config, and we would denormalize a field in the DB for external_services called has_webhooks that is used in Batch Changes to find external services without webhooks.

https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/internal/database/external_services.go?L1785%3A1-1797%3A1

We should switch to finding a matching entry in the webhooks service/table.