PERF: Add index for TopicTimer#topic_id (#17680)

When viewing a topic, we execute two queries to fetch the topic's
public topic timer and slow mode timer. The former query happens to be
able to use a unique index but the latter has to do a seq scan which is
slow. The query itself is not expensive but since viewing a topic is a
hot path, the little cuts add up overtime and the query itself
contributes significantly to the load of the database.

db/migrate/20220727040437_add_topic_timers_index.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddTopicTimersIndex < ActiveRecord::Migration[7.0]
+  def change
+    add_index :topic_timers, [:topic_id], where: "deleted_at IS NULL"
+  end
+end