Allow "Assign task" Herald Action in Maniphest to accept "None" to unassign

Summary:
Fixes T9206. This was also blocked on tokenizers being weird.

Also clean up some rendering stuff from the earlier changes.

Test Plan:

• Added an "unassign" rule by typing "None", per instructions in the placeholder text.
• Ran the rule.
• Task got unassigned.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9206

Differential Revision: https://secure.phabricator.com/D14684