Even though we do not use LFS objects, builds and build artifacts,
gitlab backups corresponding directories and recreates them upon
restore.

Current state is: the directories are living under Rails root - that is
in our gitlab-work/ inside instance, and since gitlab-work is recreated
almost on every re-instantiation if we'll have some content in that
directories - it will be lost.

Don't allow even a chance for that to happen and move what is designed
to hold data to under var/ .

NOTE we create shared/lfs-objects/ and shared/artifacts/ because if we
    do not have this dirs, gitlab backup fails. Upstream code just
    assumes the directories are there.

/cc @kazuhiko, @jerome