Use serializer to render diff lines

Move code that renders blob diff lines of app/controllers/projects/blob_controller.rb to a serializer.

Check: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17011/diffs?diff_id=18209530#d84b028799a0d15b64d62a1d547297b47bc4b58e_110_127

The following discussions from !17011 (merged) should be addressed:

• @smcgivern started a discussion:

  I think we should use a serialiser for this, but we can create a TD issue for that. Please do so and ping me there.

• @smcgivern started a discussion:

  Maybe a TD issue to add a JSON schema for these? https://gitlab.com/gitlab-org/gitlab-ce/blob/master/spec/support/api/schema_matcher.rb