Top level replies (#16087)

* DEV: Show only top level replies

Adds a new query param to the topic view so that we can filter out posts
that aren't top level replies. If a post is a reply to another post
instead of the original topic post we should not include it in the
response if the `filter_top_level_replies` query param is present.

* add rspec test

spec/requests/topics_controller_spec.rb

@@ -2428,6 +2428,29 @@ RSpec.describe TopicsController do
         end
       end
 
+      describe 'filter by top level replies' do
+        fab!(:post3) { Fabricate(:post, user: post_author3, topic: topic, reply_to_post_number: post2.post_number) }
+        fab!(:post4) { Fabricate(:post, user: post_author4, topic: topic, reply_to_post_number: post2.post_number) }
+        fab!(:post5) { Fabricate(:post, user: post_author5, topic: topic) }
+        fab!(:post6) { Fabricate(:post, user: post_author4, topic: topic, reply_to_post_number: post5.post_number) }
+
+        it 'should return the right posts' do
+          get "/t/#{topic.id}.json", params: {
+            filter_top_level_replies: true
+          }
+
+          expect(response.status).to eq(200)
+
+          body = response.parsed_body
+
+          expect(body.has_key?("suggested_topics")).to eq(false)
+          expect(body.has_key?("related_messages")).to eq(false)
+
+          ids = body["post_stream"]["posts"].map { |p| p["id"] }
+          expect(ids).to eq([post2.id, post5.id])
+        end
+      end
+
       describe 'filter upwards by post id' do
         fab!(:post3) { Fabricate(:post, user: post_author3, topic: topic) }
         fab!(:post4) { Fabricate(:post, user: post_author4, topic: topic, reply_to_post_number: post3.post_number) }