* Reword Project#latest_successful_build_for to
Project#latest_successful_build_for_ref
* Reword Ci::Pipeline#latest_successful_for to
Ci::Pipeline#latest_successful_build_for_ref

`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

It also renames the API::PROJECT_ENDPOINT_REQUIREMENTS constant to
API::NAMESPACE_OR_PROJECT_REQUIREMENTS

Partially addresses #47424.

Had to make changes to spec files because
stubbing methods on frozen objects is a mess
in RSpec and leads to failures:

https://github.com/rspec/rspec-mocks/issues/1190

This whitelists all existing offenses for the various CodeReuse cops, of
which most are triggered by the CodeReuse/ActiveRecord cop.