Support LFS fetching for CI services (ie. GitLab)

We have two possible authentication schemas:

1. User - using user credentials,
2. Project - using service credentials (mostly for CI).

We need to allow for authorize Project credentials to allow fetching LFS objects:

1. We need to pass the @ci here: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/backend/grack_auth.rb#L36
2. We need to support project authorization here: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/lfs/response.rb#L191
3. Write test to cover project authorization