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
```

2 files changed, 2 insertions, 1 deletions

diff --git a/app/models/ci/build.rb b/app/models/ci/build.rb
index 6012dbba1b9..df67fb243ad 100644
--- a/app/models/ci/build.rb
+++ b/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'
diff --git a/app/models/ci/pipeline.rb b/app/models/ci/pipeline.rb
index d4690da3be6..d7153d7b816 100644
--- a/app/models/ci/pipeline.rb
+++ b/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'