`:mailer` is needed to pick it easily, while
`type: :mailer` is needed for picking it automatically for
tests located in spec/mailers/*_spec.rb
It's a bit complicated in spec/services/notification_service_spec.rb
but we'll leave it alone for now.
1. Never send a pipeline email to anyone other than the user who created
the pipeline.
2. Only send pipeline success emails to people with the custom
notification setting for enabled. Watchers and participants will
never receive this.
3. When custom settings are unset (for new settings and legacy ones),
act as if failed_pipeline is set.
We use bcc here because we don't want to generate this emails for a
thousand times. This could be potentially expensive in a loop, and
recipients would contain all project watchers so it could be a lot.
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