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

CHANGELOG

@@ -99,6 +99,7 @@ v 8.9.0 (unreleased)
   - Remove tanuki logo from side navigation; center on top nav
   - Include user relationships when retrieving award_emoji
   - Various associations are now eager loaded when parsing issue references to reduce the number of queries executed
+  - Set inverse_of for Project/Service association to reduce the number of queries
 
 v 8.8.5 (unreleased)
   - Ensure branch cleanup regardless of whether the GitHub import process succeeds

app/models/project.rb

@@ -81,7 +81,7 @@ class Project < ActiveRecord::Base
   has_one :jira_service, dependent: :destroy
   has_one :redmine_service, dependent: :destroy
   has_one :custom_issue_tracker_service, dependent: :destroy
-  has_one :gitlab_issue_tracker_service, dependent: :destroy
+  has_one :gitlab_issue_tracker_service, dependent: :destroy, inverse_of: :project
   has_one :external_wiki_service, dependent: :destroy
 
   has_one  :forked_project_link,  dependent: :destroy, foreign_key: "forked_to_project_id"

app/models/service.rb

@@ -18,7 +18,7 @@ class Service < ActiveRecord::Base
   after_commit :reset_updated_properties
   after_commit :cache_project_has_external_issue_tracker
 
-  belongs_to :project
+  belongs_to :project, inverse_of: :services
   has_one :service_hook
 
   validates :project_id, presence: true, unless: Proc.new { |service| service.template? }