This backports to ce a refactor of the shared example 'a controller that
can serve LFS files'.

The ee MR that contains the original refactor is
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13389

The reason for the refactor was mostly the removal of the outer context
as it didn't need to be there, and prevented let(:lfs_oid) from being
overwritten.

The shared example was also renamed to be more descriptive.