Re-enable notifying commit authors of new commit notes

After upgrading to 6.0 we noticed that commit authors stopped receiving e-mail notifications about new notes to their commits. https://github.com/gitlabhq/gitlabhq/commit/cb877b7e6f6b036a2811aec7313dab94473774de#L0L109 seems to be the culprit - this probably slipped through because of missing specs. This PR fixes the issue and the specs as well.