PERF: add a filtered index for banners

This ensures we can very quickly figure out which topics are banners if
a banner is set.

Previously you would have to scan an entire table to find banners

db/migrate/20191101001705_add_banner_index_to_topics.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+class AddBannerIndexToTopics < ActiveRecord::Migration[6.0]
+  def change
+    # this speeds up the process for finding banners on the site
+    add_index :topics, [:id], name: 'index_topics_on_id_filtered_banner', where: "archetype = 'banner' AND deleted_at IS NULL", unique: true
+  end
+end