Fix PhabricatorProjectCustomFields to use storage correctly.

Summary: Prevents infinite recursion when trying to save custom fields on projects.

Test Plan: Add a custom field (that is a class, not one configured in the UI) to a project, and try to save it.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Projects: #phabricator

Maniphest Tasks: T5606

Differential Revision: https://secure.phabricator.com/D9908