Set inverse_of for Project/Services relation

This ensures that code such as this don't run needless SQL queries:

    project.gitlab_issue_tracker_service.project

This also means that if the root `project` eager loads any associations
the Service object will be able to re-use those.

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? }