FIX: Liking whispers should not contribute to Topic#like_count. (#15703)

Non-staff users are not allowed to see whisper so this change prevents
non-staff user from seeing a like count that does not make sense to
them. In the future, we might consider adding another like count column
for staff user.

Follow-up to 4492718864

spec/models/post_action_spec.rb

@@ -456,6 +456,16 @@ describe PostAction do
       expect(notification.notification_type).to eq(Notification.types[:liked])
     end
 
+    it 'should not increase topic like count when liking a whisper' do
+      SiteSetting.set(:enable_whispers, true)
+      post.revise(admin, post_type: Post.types[:whisper])
+
+      PostActionCreator.like(admin, post)
+
+      expect(post.reload.like_count).to eq(1)
+      expect(post.topic.like_count).to eq(0)
+    end
+
     it 'should increase the `like_count` and `like_score` when a user likes something' do
       freeze_time Date.today