Signed-off-by: NRémy Coutable <remy@rymai.me>

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.

[ci skip]

This change simplifies a Pipeline processing by introducing a special new status: created.
This status is used for all builds that are created for a pipeline.
We are then processing next stages and queueing some of the builds (created -> pending) or skipping them (created -> skipped).
This makes it possible to simplify and solve a few ordering problems with how previously builds were scheduled.
This also allows us to visualise a full pipeline (with created builds).

This also removes an after_touch used for updating a pipeline state parameters.
Right now in various places we explicitly call a reload_status! on pipeline to force it to be updated and saved.