Move Curated pool updater to Danbooru.

* Move the Curated pool updater from Reportbooru to Danbooru.

* Change the process for selecting curated posts. Previously it was
  every post from the last week with at least three supervotes. This was
  flawed because it included both super-upvotes and super-downvotes. Now
  it's the top 100 posts from the last week, ordered from most super-upvoted
  to least.

test/unit/danbooru_maintenance_test.rb

@@ -3,13 +3,20 @@ require 'test_helper'
 class DanbooruMaintenanceTest < ActiveSupport::TestCase
   context "daily maintenance" do
     setup do
-      FactoryBot.create(:admin_user) # for SuperVoter.init!
+      @admin = create(:admin_user) # for SuperVoter.init!
     end
 
     should "work" do
       assert_nothing_raised { DanbooruMaintenance.daily }
     end
 
+    should "update the curated pool" do
+      pool = as(@admin) { create(:pool, name: "curated") }
+      Danbooru.config.stubs(:curated_pool_id).returns(pool.id)
+
+      assert_nothing_raised { DanbooruMaintenance.daily }
+    end
+
     should "prune expired posts" do
       @pending = FactoryBot.create(:post, is_pending: true, created_at: 4.days.ago)
       @flagged = FactoryBot.create(:post, is_flagged: true, created_at: 4.days.ago)