Integrate Git LFS file locking with GitLab file locking

Git LFS has released file locking that is accessed through the Git LFS CLI. GitLab supports file locking, but only through the web interface.

We should make the GitLab web interface file locking and Git LFS file locking work together.

Proposal

• Add LFS file locking API (https://gitlab.com/gitlab-org/gitlab-ce/issues/35856)
• Integrate LFS file locking with GitLab file locking interface so that
  • A LFS file locked or unlocked through the GitLab interface is reported as locked/unlocked via the LFS API
  • A LFS file locked or unlocked through the LFS locking API is reported as locked/unlocked via the GitLab interface

Source

Git LFS locking released: https://github.com/blog/2328-git-lfs-2-0-0-released Git LFS locking API: https://github.com/git-lfs/git-lfs/blob/master/docs/api/locking.md