PERF: Fix N+1 queries when serializing topic posters (#19545)

At the time of writing, this is how the `TopicPosterSerializer` looks
like:

```
class TopicPosterSerializer < ApplicationSerializer
  attributes :extras, :description

  has_one :user, serializer: PosterSerializer
  has_one :primary_group, serializer: PrimaryGroupSerializer
  has_one :flair_group, serializer: FlairGroupSerializer
end
```

Within `PosterSerializer`, the `primary_group` and `flair_group`
association is requested on the `user` object. However, the associations
have not been loaded on the `user` object at this point leading to the
N+1 queries problem. One may wonder
why the associations have not been loaded when the `TopicPosterSerializer`
has `has_one :primary_group` and `has_one :flair_group`. It turns out that `TopicPoster`
is just a struct containing the  `user`, `primary_group` and
`flair_group` objects. The `primary_group` and `flair_group`
ActiveRecord objects are loaded seperately in `UserLookup` and not preloaded when querying for
the users. This is done for performance reason so that we are able to
load the `primary_group` and `flair_group` records in a single query
without duplication.

spec/requests/list_controller_spec.rb

@@ -99,6 +99,46 @@ RSpec.describe ListController do
       expect(first_item.css('[itemprop="position"]')[0]['content']).to eq('1')
       expect(first_item.css('[itemprop="url"]')[0]['href']).to eq(topic.url)
     end
+
+    it 'does not N+1 queries when topic featured users have different primary groups' do
+      user.update!(primary_group: group)
+
+      # warm up
+      get "/latest.json"
+      expect(response.status).to eq(200)
+
+      initial_sql_queries_count = track_sql_queries do
+        get "/latest.json"
+
+        expect(response.status).to eq(200)
+
+        body = response.parsed_body
+
+        expect(body["topic_list"]["topics"].map { |t| t["id"] }).to contain_exactly(topic.id)
+        expect(body["topic_list"]["topics"][0]["posters"].map { |p| p["user_id"] }).to contain_exactly(user.id)
+      end.count
+
+      group2 = Fabricate(:group)
+      user2 = Fabricate(:user, primary_group: group2)
+      topic.update!(last_post_user_id: user2.id)
+
+      group3 = Fabricate(:group)
+      user3 = Fabricate(:user, flair_group: group3)
+      topic.update!(featured_user3_id: user3.id)
+
+      new_sql_queries_count = track_sql_queries do
+        get "/latest.json"
+
+        expect(response.status).to eq(200)
+
+        body = response.parsed_body
+
+        expect(body["topic_list"]["topics"].map { |t| t["id"] }).to contain_exactly(topic.id)
+        expect(body["topic_list"]["topics"][0]["posters"].map { |p| p["user_id"] }).to contain_exactly(user.id, user2.id, user3.id)
+      end.count
+
+      expect(new_sql_queries_count).to be <= initial_sql_queries_count
+    end
   end
 
   describe "categories and X" do