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.
Updates specs to use new rails5 format.
The old format:
`get :show, { some: params }, { some: headers }`
The new format:
`get :show, params: { some: params }, headers: { some: headers }`
This would result in a 404 error in certain views.
The `save` call was being rolled back due to an error in the validation step.
Relax the validation step so that this works.
Closes#1570
