Upstreaming from https://phabricator.wikimedia.org/T393464.
This is on top of T16058: Workboard column triggers: Allow assigning to user performing the move / D25991: Column triggers: Allow assigning user who performs the move.

Current trigger rules don't allow setting any conditions for when to trigger so this requires a new trigger type to ignore resetting the assignee on already assigned tasks when moving to a column with such a trigger: "Assign unassigned task to..."