Reduce number of queries made for merge_requests/:id/diffs

## What does this MR do?
It reduces the number of DB queries made while processing and rendering MR notes.

## Are there points in the code the reviewer needs to double check?
N/A

## Why was this MR needed?
For `https://staging.gitlab.com/gitlab-org/gitlab-ce/merge_requests/3142/diffs.json`, for each note we make number of DB queries, almost all of them are handled by the AR caching layer, but they seem to add up a few seconds. Testing on staging, calling `merge_requests/3142/diffs.json` was reduced to ~5.5 seconds from ~8 seconds.

## What are the relevant issue numbers?
N/A

## Screenshots (if relevant)
N/A

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [ ] ~~API support added~~
- ~~Tests~~
  - [ ] ~~Added for this feature/bug~~
  - [ ] ~~All builds are passing~~
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5551