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 apply the settings in the UI.

Test Plan:

• Viewed revisions, changed highlighting, reloaded. Saw highlighting stick in revision view and standalone view.
• Viewed commits, changed highlighting, reloaded. Saw highlighting stick.
• Created a diff, changed highlighting, turned it into a revision, saw highlighting persist.

Subscribers: jmeador, PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13455

Differential Revision: https://secure.phabricator.com/D21137