webhooks: initial prototype for GQL-native webhooks

database.WithTx: rollback if func returns error

Previously, this only rolled back if the called function panicked. This
updates it to also roll back if the called function returns an error.

webhooks/legacy_test: verify headers are recorded

Deduplicate webhook headers on subsequent attempts

This also updates the database with the final payload headers, including
the signature and nonce, which are generated afresh for each attempt.

webhooks/legacy: increase specificity of logging

workers/legacy: correct Println with fmt directive

Make worker logs less loud

webhooks/legacy: address Postgres-specific issues

webhooks/legacy_test.go: style

webhooks/legacy: update only affected delivery

Forgot to add this where clause, whoops!

Add legacy webhooks worker implementation