Add a way to flag a topic

app/controllers/post_actions_controller.rb

@@ -12,6 +12,7 @@ class PostActionsController < ApplicationController
     args = {}
     args[:message] = params[:message] if params[:message].present?
     args[:take_action] = true if guardian.is_staff? and params[:take_action] == 'true'
+    args[:flag_topic] = true if params[:flag_topic]
 
     post_action = PostAction.act(current_user, @post, @post_action_type_id, args)
 
@@ -63,7 +64,18 @@ class PostActionsController < ApplicationController
 
     def fetch_post_from_params
       params.require(:id)
-      finder = Post.where(id: params[:id])
+
+      post_id = if params[:flag_topic]
+        begin
+          Topic.find(params[:id]).posts.first.id
+        rescue
+          raise Discourse::NotFound
+        end
+      else
+        params[:id]
+      end
+
+      finder = Post.where(id: post_id)
 
       # Include deleted posts if the user is a moderator (to guardian ?)
       finder = finder.with_deleted if current_user.try(:moderator?)