Link to latest pipeline and job (Backend preparation)

Description

Introduce http://gitlab.com/<Namespace>/pipelines/latest endpoint which shows the latest pipeline on a branch.

For exmaple, you can see the latest pipeline on a feature-1 branch with http://gitlab.com/<Namespace>/pipelines/latest?ref=feature-1

If you don't specify the ref, you see the latest pipeline on a default branch.

This is a prelinimary work for https://gitlab.com/gitlab-org/gitlab-ce/issues/39099 and will be polished in the future iteration with ~UX involvments

Screenshots

TBD

/cc @alexives