R1:7e1743a959e0

Add a trigger rule to reassign a task

Summary:
Ref T13269. Workboard triggers can now reassign tasks on column drop. Also sprinkles some `setViewer()` calls in places that needed them.

This mostly works, but a few issues:

* To set the owner to unassigned, you must explicitly put the "No Owner" token in the typeahead. Maybe this should just figure out you've put nothing in that field and set it for you?
* I'm pretty sure this was already broken, but if you change the rule type from a tokenizer to a…