Copy LFS objects when a repository is mirrored

Projects can be mirrored from or to another repository. When activated, we mirror branches, tags, and commits. However, we do not mirror the LFS objects if the repository uses LFS. This is documented in a few places but is certainly not an expected or obvious behaviour and makes mirroring unusable for repositories containing LFS objects.

We should add support for mirroring LFS objects.

Proposal

• Include git-lfs with GitLab (required by gitlab-org/gitlab-ce#40401)
• Mirror LFS objects for push and pull mirroring
• Remove documentation explaining we don't support LFS object mirroring
  • Documentation https://gitlab.com/gitlab-org/gitlab-ee/issues/1291
  • Mirror project settings screen

References

Issue that mentioned this problem: https://gitlab.com/gitlab-org/gitlab-ee/issues/1252