DEV: Migrate sidebar site settings (#19336)

This new site setting replaces the
`enable_experimental_sidebar_hamburger` and `enable_sidebar` site
settings as the sidebar feature exits the experimental phase.

Note that we're replacing this without depreciation since the previous
site setting was considered experimental.

Internal Ref: /t/86563

db/migrate/20221205225450_migrate_sidebar_site_settings.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+class MigrateSidebarSiteSettings < ActiveRecord::Migration[7.0]
+  def up
+    previous_enable_experimental_sidebar_hamburger = DB.query_single(
+      "SELECT value FROM site_settings WHERE name = 'enable_experimental_sidebar_hamburger'"
+    )[0]
+
+    previous_enable_sidebar = DB.query_single(
+      "SELECT value FROM site_settings WHERE name = 'enable_sidebar'"
+    )[0]
+
+    value =
+      case [previous_enable_experimental_sidebar_hamburger, previous_enable_sidebar]
+      when ['t', 't'], ['t', nil]
+        "sidebar"
+      when ['t', 'f']
+        "header dropdown"
+      when ['f', 't'], ['f', 'f'], ['f', nil]
+        "legacy"
+      when [nil, 't'], [nil, 'f'], [nil, nil]
+        nil
+      end
+
+    if value
+      execute(<<~SQL)
+      INSERT INTO site_settings (name, data_type, value, created_at, updated_at)
+      VALUES ('navigation_menu', 8, '#{value}', now(), now())
+      SQL
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end