Respect tokenizer limits in the bulk editor

Summary: Ref T13025. This makes limits (for fields like "Assign To") work in the bulk editor, so you can't type "Assign to: x, y, z" anymore.

Test Plan: Hit limit for "Assign to" and a custom project field. No limit for "Add subscribers".

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13025

Differential Revision: https://secure.phabricator.com/D18888