DEV: Add reviewables tab to the new user menu (#17630)

This commit is a subset of the changes proposed in https://github.com/discourse/discourse/pull/17379.

db/migrate/20220727085001_create_index_on_reviewables_score_desc_created_at_desc.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class CreateIndexOnReviewablesScoreDescCreatedAtDesc < ActiveRecord::Migration[7.0]
+  def change
+    add_index(
+      :reviewables,
+      [:score, :created_at],
+      order: { score: :desc, created_at: :desc },
+      name: 'idx_reviewables_score_desc_created_at_desc'
+    )
+  end
+end