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

resources/sql/autopatches/20200417.viewstate.01.storage.sql

@@ -0,0 +1,9 @@
+CREATE TABLE {$NAMESPACE}_differential.differential_viewstate (
+  id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
+  viewerPHID VARBINARY(64) NOT NULL,
+  objectPHID VARBINARY(64) NOT NULL,
+  viewState LONGTEXT NOT NULL COLLATE {$COLLATE_TEXT},
+  dateCreated INT UNSIGNED NOT NULL,
+  dateModified INT UNSIGNED NOT NULL,
+  UNIQUE KEY `key_viewer` (viewerPHID, objectPHID)
+) ENGINE=InnoDB, COLLATE {$COLLATE_TEXT};