Merge branch 'eager-loading-issue-parser' into 'master'

Eager load project relations in IssueParser

## What does this MR do?

This changes the ReferenceParser class to eager load various associations. This in turn results in the permissions checking code (e.g. the `Ability` model) to _not_ run dozens if not hundreds of extra SQL queries depending on the amount of references involved (in a single document).

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

No.

## Why was this MR needed?

In !4410 it was revealed a _lot_ of a queries came from the `Ability` model and the code it would call. In many cases this was because the code would simply get a project, then get the owners; or get a group, then get some association of that. Eager loading these associations is a fairly simple solution and greatly cuts down the number of queries.

## What are the relevant issue numbers?

None.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [x] ~~API support added~~
- [ ] Tests
  - [x] ~~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)
- [ ] 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 !4675

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