FEATURE: Topic slow mode. (#10904)

Adds a new slow mode for topics that are heating up. Users will have to wait for a period of time before being able to post again.

We store this interval inside the topics table and track the last time a user posted using the last_posted_at datetime in the TopicUser relation.

app/controllers/topics_controller.rb

@@ -28,7 +28,8 @@ class TopicsController < ApplicationController
     :convert_topic,
     :bookmark,
     :publish,
-    :reset_bump_date
+    :reset_bump_date,
+    :set_slow_mode
   ]
 
   before_action :consider_user_for_promotion, only: :show
@@ -932,6 +933,15 @@ class TopicsController < ApplicationController
     render body: nil
   end
 
+  def set_slow_mode
+    topic = Topic.find(params[:topic_id])
+
+    guardian.ensure_can_moderate!(topic)
+    topic.update!(slow_mode_seconds: params[:seconds])
+
+    head :ok
+  end
+
   private
 
   def topic_params