Fix value reading in custom Text and Remarkup fields in EditEngine

Summary: Ref T10004. I missed these previously, so they didn't work quite right. Restore them to glory.

Test Plan: Edited remarkup and text custom fields on an Owners package.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10004

Differential Revision: https://secure.phabricator.com/D14821