`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.
Add a new endpoint
`projects/:id/jobs/artifacts/:ref_name/raw/*artifact_path?job=name`
which is the close the web URL for consistency sake. This endpoint can
be used to download a single file from artifacts for the specified ref
and job.
closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54626
