Loading merge requests with lots of notes is really slow

Prime example: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2987 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5293

This is probably due to the Markdown pipeline having to process a lot of notes and probably running quite a few queries in the process. I've assigned this to 8.7 since we're not really sure just yet what's causing this and if it indeed is the Markdown pipeline there's probably quite a bit we have to change to improve things.