Always send a failed/success pipeline email (except when it becomes a fixed pipeline)

Following the discussion from !26803 (comment 303727766)

• When it's a failed/success pipeline, always send an email, regardless it's retried or ancient to the appropriate subscribers
• If the success pipeline will become a fixed pipeline, upgrade the success email to be a fixed email, and send to appropriate subscribers. A fixed pipeline is also considered a success pipeline, so a fixed email should also be sent to subscribers where they subscribed to success pipelines.

This is still changing the behaviour because we upgrade the success email to be a fixed email. If we want to fully preserve the behaviour, we'll need to send both fixed email and success email when a pipeline is fixed to the corresponding subscribers, so people subscribing to success pipeline will only receive success email, and people subscribing to fixed pipeline will only receive fixed email.

/cc @dosuken123 @jyavorska @jacopo-beschi