R1:8aac55cc57cb

Make "Highlight As..." sticky across reloads in Diffusion and Differential

Summary:
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…