Add JSON schema specs for internal APIs
We have some internal JSON APIs, that live in Rails controllers. @dbalexandre has added a JSON schema validator in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5548. We should go back and add this validation to our existing internal APIs, to ensure we don't break things in future. As we add more front-end code using Vue, this will become more important.
Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/20387, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2397