Simplify user cache management of data forms

Summary: Ref T4103. Ref T10078. We currently have separate "usable" and "raw" values, but can simplify this by making newValueForUsers() return the raw value.

Test Plan: Ran unit tests; browsed around; dropped caches and browsed around.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103, T10078

Differential Revision: https://secure.phabricator.com/D16043