R1:91a78db99b33

Support "Assign To" in Maniphest bulk editor

Summary:
Ref T13025. See PHI173. This supports the "Assign to" field in the new editor.

This is very slightly funky: to unassign tasks, you need to leave the field blank. I have half a diff to fix this, but the way the `none()` token works in the default datasource is odd so it needs a separate datasource. I'm punting on this for now since it works, at least, and isn't //completely// unreasonable.

This also simplifies some EditEngine stuff a little. Notably…