Fix reading ad-hoc settings with no formal Settings object

Summary:
Ref T4103. Some settings (like the collapsed/expanded state of the diff filetree) are currently ad-hoc. They weren't being read correctly.

Also, simplify the caching code a little bit.

Test Plan: Toggled filetree, reloaded page, got sticky behavior.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D16021