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