Work around an issue with custom "users" fields in Maniphest

Summary:
Fixes T9558. The recent changes to validate PHID fields don't work cleanly with this gross hack.

This can probably be unwound now but it will definitely get fixed in T9132 so I may just wait for that.

Test Plan: Edited a custom "users" field in Maniphest. This should only affect Maniphest because it has a weird hack.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9558

Differential Revision: https://secure.phabricator.com/D14264