Merge branch 'jimmykarily/gitlab-ce-notify_current_user_when_merging_an_mr_after_build_succeeds' into 'master'

Notify current_user about automatic merge after successful build

It enables notifications to the initiator of a merge when the MR is flagged as "Merge when build succeeds".

Because when running Builds, quite some time passes between the user's
action and the actual Merge so it is a good thing to notify the
initiator of the Merge when it actually happens.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14409

See merge request !6534