To avoid having unassigned work in progress, we should have the ability to create new triggers under the columns, similar to what exists now, but instead of assigning to a concrete user, we should be able to assign to the person who has performed the column movement.
Context: https://www.mediawiki.org/wiki/Phabricator/Project_management#Automated_actions_via_column_triggers
Upstream: https://we.phorge.it/T16058