Refactor project.latest_successful_builds_for def
`project.latest_successful_builds_for(ref)` is being used to find a single job all the time. This results into us having to call `find_by` inside of the controller which violates our CodeReuse/ActiveRecord rubocop rule. Refactor `project.latest_successful_builds_for(ref)` to `project.latest_successful_build_for(job_name, ref)` which will execute the `find_by` inside of the model. Also create `project.latest_successful_build_for!(job_name, ref)` which raises an exception instead of returning nil.