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
```