Package the notifiers for releaes

[leehambley]

The notifiers (email, slack, and job (e.g trigger another job)) need much work before they can be released publicly, they don't build at all without a lot of infrastructure that is internal to us, and they're mandatory for a fully working version of Harrow that doesn't break down.

Similarly the existing code in cmd/operation-runner/spawn.go refers to ...%s controller-shell -operation-uuid %s '/srv/harrow/bin/harrow-notify-${NOTIFIER_TYPE}'" ... which under the new arrangement of code is no longer valid and must rely on the $PATH.