Ref T13455. Add container-level storage for persistent view state, and persist "Highlight As..." inside it.
The storage generates a "PhabricatorChangesetViewState" configuration object as an output.
When preferences are expressed on a diff and that diff is later attached to a revision, we attempt to copy the preferences.
The internal storage tracks per-changeset settings, but currently always uses "last update wins" to apply the settings in the UI.