Moving from https://github.com/catapult-project/catapult/issues/2582

Occasionally, sheriffs may assign regression alerts to one bug, then re-assign them to a different bug. Changing the set of alerts that are assigned to a bug may confuse other sheriffs.
When alerts are re-triaged like this, the dashboard should post an update to the initial bug containing the username of the sheriff and a link to the new bug. If the set of alerts assigned to a bug becomes empty, it should automatically be closed.