allowing override for EE.

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.

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6342#note_16132150