Merge branch 'project-services-inverse-of' into 'master'

Set inverse_of for Project/Services relation

## What does this MR do?

This MR adds the `inverse_of:` option to two associations to reduce the number of queries when running code such as ` project.gitlab_issue_tracker_service.project`.

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

No.

## Why was this MR needed?

In !4410 it was revealed code such as the above is used and would run SQL queries when the root object (usually a Project) was already present. By using `inverse_of` Rails can just re-use those Project instances.

## 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~~
- [x] Tests
  - [x] ~~Added for this feature/bug~~
  - [x] 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 !4674

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