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:

• I reorganized EditType construction slightly so subclasses can copy/paste a little bit less.
• EditType had field and editField properties which had the same values. I canonicalized on editField and made this value set a little more automatically.

Test Plan: Used bulk editor to reassign some tasks. By leaving the field blank, unassigned tasks.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13025

Differential Revision: https://secure.phabricator.com/D18874