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