Ensure CI pipelines and builds have the correction project associations

This should reduce the number of SQL queries and lookups needed to look up a
project of a build and pipeline and vice versa.

Before:

```
[1] pry(main)> Ci::Build.reflect_on_association(:project).has_inverse?
=> false
[2] pry(main)> Project.reflect_on_association(:builds).has_inverse?
=> false
[3] pry(main)> Ci::Pipeline.reflect_on_association(:project).has_inverse?
=> false
[4] pry(main)> Project.reflect_on_association(:pipelines).has_inverse?
=> :project
```

After:

```
[1] pry(main)> Ci::Build.reflect_on_association(:project).has_inverse?
=> :builds
[2] pry(main)> Project.reflect_on_association(:builds).has_inverse?
=> :project
[3] pry(main)> Ci::Pipeline.reflect_on_association(:project).has_inverse?
=> :pipelines
[4] pry(main)> Project.reflect_on_association(:pipelines).has_inverse?
=> :project
```

app/models/ci/build.rb

@@ -8,6 +8,7 @@ module Ci
 
     MissingDependenciesError = Class.new(StandardError)
 
+    belongs_to :project, inverse_of: :builds
     belongs_to :runner
     belongs_to :trigger_request
     belongs_to :erased_by, class_name: 'User'

app/models/ci/pipeline.rb

@@ -7,7 +7,7 @@ module Ci
     include Presentable
     include Gitlab::OptimisticLocking
 
-    belongs_to :project
+    belongs_to :project, inverse_of: :pipelines
     belongs_to :user
     belongs_to :auto_canceled_by, class_name: 'Ci::Pipeline'
     belongs_to :pipeline_schedule, class_name: 'Ci::PipelineSchedule'

app/models/project.rb

@@ -199,13 +199,13 @@ class Project < ActiveRecord::Base
   has_many :container_repositories, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
 
   has_many :commit_statuses
-  has_many :pipelines, class_name: 'Ci::Pipeline'
+  has_many :pipelines, class_name: 'Ci::Pipeline', inverse_of: :project
 
   # Ci::Build objects store data on the file system such as artifact files and
   # build traces. Currently there's no efficient way of removing this data in
   # bulk that doesn't involve loading the rows into memory. As a result we're
   # still using `dependent: :destroy` here.
-  has_many :builds, class_name: 'Ci::Build', dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
+  has_many :builds, class_name: 'Ci::Build', inverse_of: :project, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
   has_many :build_trace_section_names, class_name: 'Ci::BuildTraceSectionName'
   has_many :runner_projects, class_name: 'Ci::RunnerProject'
   has_many :runners, through: :runner_projects, source: :runner, class_name: 'Ci::Runner'